Ukraine says a political ally of Putin has been detained in a ‘special operation.’

Ukraine’s security service said on Tuesday that officers had detained Viktor Medvedchuk, a politician and oligarch seen as the Kremlin’s main agent of influence in Ukraine in recent years.

Mr. Medvedchuk disappeared shortly after the start of the war and the Ukrainian authorities were searching for him.

Ivan Bakanov, the head of the Security Service of Ukraine, said on Facebook that officers had conducted a “lightning-fast and dangerous multilevel special operation” to detain Mr. Medvedchuk.

Dmitri S. Peskov, the Kremlin’s spokesman, said he could not confirm that Mr. Medvedchuk had been arrested, according to Russian state media. President Volodymyr Zelensky of Ukraine posted a photo on Telegram that appeared to show Mr. Medvedchuk in handcuffs.

Mr. Medvedchuk’s arrest would be a significant blow to President Vladimir V. Putin of Russia, who is reported to be the godfather to Mr. Medvedchuk’s daughter and counts him as one of his closest allies in Ukraine.

A longtime influential player in politics and business between Ukraine and Russia, he was an adviser in the office of President Leonid Kuchma after Ukraine’s independence and took part in negotiations with separatists in eastern Ukraine after Russia’s annexation of Crimea.

Before the war, Mr. Medvedchuk had been under house arrest on suspicion of treason, attempted looting of national resources and aiding and abetting a terrorist organization, according to local media.

A Ukrainian official, Mykhailo Podolyak, a member of Ukraine’sdelegation for peace talks in Istanbul, said on Twitter that Mr. Medvedchuk had “regularly lied about the situation” in Ukraine, and “stole money and eventually became one of the initiators of the war.”

Oleksandr Chubko contributed reporting from Kyiv, Ukraine.