KYIV, Ukraine — President Volodymyr Zelensky of Ukraine agreed on Sunday to talks with Russia “without preconditions,” even as President Vladimir V. Putin escalated tensions by placing his nuclear forces on alert.
Mr. Zelensky said that he would send a delegation to meet with Russian officials near the border with Belarus, at a still undetermined time, but that he would remain in Kyiv, with his top officials.
“I do not really believe in the outcome of this meeting,” he said, “but let them try to make sure that no citizen of Ukraine has any doubt that I, as a president, have not tried to stop the war.”
Ukrainian officials took some obvious satisfaction in Russia’s call for talks, which came as its forces met far more resistance than expected, failing to quickly seize the capital, Kyiv.
“The enemy expected an easy walk, but got real hell,” said Prime Minister Denys Shmygal. Russia’s leaders, he said, “does not understand that it is at war not only with the armed forces of Ukraine, but with the entire Ukrainian people.”
The request for talks also came as the European Union moved to impose tough new economic sanctions on Russia, and as demonstrations in Berlin, Prague, London, Madrid and Brussels over the weekend on behalf of Ukrainians made Moscow’s isolation clear.
But international military experts cautioned that the war is young.
They noted that Ukrainian forces are spread thin, with only limited ammunition, and that thousands of better-trained Russian soldiers have not yet been thrown into the fight. The worry is that Mr. Putin may move to harsher tactics, including the shelling of cities, if his forces get bogged down.
Mr. Putin’s new nuclear threat appeared to be aimed at the West, which has increasingly rallied behind Ukraine. In brief remarks aired on state television, he told his defense minister and his top military commander to place Russia’s nuclear forces on alert, characterizing the move as a response to the West’s “aggressive” actions.
Not only are Western countries imposing “illegitimate sanctions” against Russia, Mr. Putin said, “but senior officials of leading NATO countries are allowing themselves to make aggressive statements directed at our country.”
The White House, through Jen Psaki, the press secretary, cast the nuclear alert as another example of Mr. Putin’s manufacturing threat and using it to justify confrontation.
Top Pentagon leaders remain confident about the United States’ ability to defend itself and its allies, according to a Pentagon official who briefed reporters. The official called the move to put Russia’s nuclear forces on alert unnecessary and escalatory, and said that Mr. Putin had made the possibility of a miscalculation much more dangerous.
Slowed by Ukrainian resistance and their own logistical shortcomings, Russian forces have already begun adoptingharsher methods such as rocket attacks in the city of Chernihiv, to the northeast of Kyiv, the Pentagon official said. Those tactics could produce many more civilian casualties.
Russian troops, he said, are adopting a siege mentality, which increases the likelihood of their taking civilian life and damaging infrastructure.
In Ukraine’s south, the official said, Russian troops are advancing, moving up from Crimea and also mounting an amphibious assault near Mariupol, where they are within 50 kilometers of the city. Mariupol, a major port, is viewed as part of any Russian land bridge from Donetsk and Luhansk — the eastern Ukrainian regions Moscow just recognized as independent states — to Crimea, which Moscow annexed in 2014.
Russian forces were also on the move from the north, where they were making a drive toward Kyiv.
In the center of Ukraine, Russia appears to be trying to cut off the main Ukrainian military forces, which have been defending the former line of contact with the Donetsk and Luhansk enclaves, to prevent them from moving toward the capital and getting supplies sent by Western allies overland through Poland.
Russian troops, at least for a time, also drew closer to the center of Kharkiv, Ukraine’s second-largest city, according to videos and photographs analyzed by The New York Times. The footage showed Ukrainians firing rockets toward Russian troops, as well as some Russian military vehicles burning and others being ransacked by Ukrainian forces.
With each passing day of war, Mr. Zelensky, a former comic, has embraced his new role: as a symbol of bravery and patriotism who has united his citizens against an invader.
Once derided because of his entertainment past as the unlikeliest of presidents, Mr. Zelensky has transformed into the leader Ukraine did not know it needed. Dressed in an army-green T-shirt or fleece, unshaven and wan, he has inspired Ukrainians to defend their streets with the most rudimentary of weapons. He has also won over much of Europe, which has been moved to aid a country it sees as fighting bravely for independence, freedom and democracy.
Mr. Zelensky’s decision to remain in Kyiv — and his family’s decision to remain in Ukraine — has moved many. Some have drawn an unflattering contrast with the Afghan president, Ashraf Ghani, who fled Kabul as soon as the Taliban were on the outskirts, demoralizing what was left of the Afghan army.
And Mr. Zelensky’s response to a reported American offer to evacuate him — “I need ammunition, not a ride” — will most likely go down in Ukrainian history, whatever the outcome of the battle.
The Ukrainian president and his team have made deft use of social media. His impassioned speeches from the streets of Kyiv have gone viral. And they have also posted photos and short videos of Ukrainians filling Molotov cocktails, volunteering to fight, being issued automatic weapons and vowing to defend their country.
Mr. Zelensky has also inspired European leaders to do more to help his country. His leadership and the resilience of the Ukrainian people “are an inspiration to us all,” said Ursula von der Leyen, president of the European Commission.
Understand Russia’s Attack on Ukraine
What is at the root of this invasion? Russia considers Ukraine within its natural sphere of influence, and it has grown unnerved at Ukraine’s closeness with the West and the prospect that the country might join NATO or the European Union. While Ukraine is part of neither, it receives financial and military aid from the United States and Europe.
Are these tensions just starting now? Antagonism between the two nations has been simmering since 2014, when the Russian military crossed into Ukrainian territory, after an uprising in Ukraine replaced their Russia-friendly president with a pro-Western government. Then, Russia annexed Crimea and inspired a separatist movement in the east. A cease-fire was negotiated in 2015, but fighting has continued.
How did this invasion unfold? After amassing a military presence near the Ukrainian border for months, on Feb. 21, President Vladimir V. Putin of Russia signed decrees recognizing two pro-Russian breakaway regions in eastern Ukraine. On Feb. 23, he declared the start of a “special military operation” in Ukraine. Several attacks on cities around the country have since unfolded.
What has Mr. Putin said about the attacks? Mr. Putin said he was acting after receiving a plea for assistance from the leaders of the Russian-backed separatist territories of Donetsk and Luhansk, citing the false accusation that Ukrainian forces had been carrying out ethnic cleansing there and arguing that the very idea of Ukrainian statehood was a fiction.
How has Ukraine responded? On Feb. 23, Ukraine declared a 30-day state of emergency as cyberattacks knocked out government institutions. Following the beginning of the attacks, Volodymyr Zelensky, Ukraine’s president, declared martial law. The foreign minister called the attacks “a full-scale invasion” and called on the world to “stop Putin.”
How has the rest of the world reacted? The United States, the European Union and others have condemned Russia’s aggression and begun issuing economic sanctions against Russia. Germany announced on Feb. 23 that it would halt certification of a gas pipeline linking it with Russia. China refused to call the attack an “invasion,” but did call for dialogue.
How could this affect the economy? Russia controls vast global resources — natural gas, oil, wheat, palladium and nickel in particular — so the conflict could have far-reaching consequences, prompting spikes in energy and food prices and spooking investors. Global banks are also bracing for the effects of sanctions.
Appearing onscreen during than an emergency summit meeting of European Union leaders several days ago, he gave a passionate 10-minute speech that moved some reluctant leaders to endorse a harsher package of economic sanctions on Russia, said a senior European official who was in the room.
“This may be the last time you see me alive,” Mr. Zelensky told them.
The silence in the room after Mr. Zelensky spoke was impressive, the official said, and it was his impression the speech made a major difference in convincing more reluctant countries, like Germany, Italy and Hungary, to agree to tougher financial and banking sanctions and to deliver defensive weapons to Ukraine.
The new government of Germany also made what some consider a historic shift toward taking more responsibility for European security. Hampered by its own totalitarian history, Germany has been a reluctant hard-power player since the collapse of the Soviet Union. But on Sunday, in a strong speech before the German Parliament, Chancellor Olaf Scholz turned the page.
Germany will send defensive weapons to Ukraine; support tougher economic sanctions, including the exclusion of major Russian banks from the SWIFT payments system; and raise military spending to more than 2 percent of its gross domestic product for the foreseeable future — long a NATO goal that Germany had resisted. Mr. Scholz also proposed a 100 billion euro fund to strengthen the German army.
Mr. Scholz, a member of the Social Democratic Party, which has been known for its closeness to Moscow, also said that Germany would agree to replace its aging Tornado fighters with newer planes capable of dropping nuclear bombs. He already had broken with previous German policy by agreeing to put on hold theNord Stream 2 natural gas pipeline, which goes from Russia to Germany, bypassing Ukraine.
Mr. Scholz, German officials say, believes that Mr. Putin lied to him personally in their direct talks in Moscow about the Ukraine crisis and that the Russian invasion created serious new threats to German and European security that cannot be ignored.
Claudia Major, a defense expert with the German Institute for International and Security Affairs, called the changes “a revolution in German defense policy,’’ while Jana Puglierin, director of the Berlin office of the European Council on Foreign Relations, said: “It is hard to overestimate the significance of this. I would not have thought it possible.”
In an emergency virtual meeting of European Union foreign ministers on Sunday, the bloc agreed to further tough economic sanctions on Russia and to ban Russian aircraft from E.U. bloc’s airspace. They also agreed for the first time to use E.U. funding to reimburse member countries for the purchase and shipment of military equipment to a country under attack.
Ms. von der Leyen, the commission president, called it “a watershed moment.”
Weapons and equipment of all kinds are being sent overland through Poland into western Ukraine, which is still out of Russia’s reach. Some experts think Russian efforts to disrupt such convoys will increase once the shipments are inside Ukraine.
Ms. von der Leyen also said that the European Union would move to ban the state-owned news outlets Russia Today and Sputnik, so they “will no longer be able to spread their lies to justify Putin’s war and to sow division.”
Europe will continue to welcome refugees, she said. As many as sevenmillion Ukrainians are internally displaced by the conflict, European officials said.
In Kyiv, Nataly Kasianenko, 31, has been sheltering for days with her husband and a dachshund in a parking garage in the northern Obolon neighborhood, six miles from the city center, which was the scene of intense fighting on Friday and Saturday. “I actually lost track of days and nights and nights,’’ she said. “We even forgot to eat because of this tension.”
Despite the fear, she said, they are staying.
“It’s in our blood, it’s Ukrainian blood,” she said. “We can’t just leave, we cannot just surrender — we will always stay on our land.”
Valerie Hopkins reported from Kyiv, Anton Troianovski from Moscow and Steven Erlanger from Brussels. Andrew E. Kramer, Michael Schwirtz, Marc Santora, Helene Cooper, Aishvarya Kavi, Matina Stevis-Gridneff, Monika Pronczuk, Melissa Eddy contributed reporting.