A U.S. diplomat says Russia is planning to annex parts of Ukraine with ‘sham’ elections.

WASHINGTON — Russia appears to be preparing to annex two regions in eastern Ukraine and possibly a third in the country’s south, a senior American diplomat said on Monday, citing “highly credible” reports of Moscow’s plans.

Michael Carpenter, the American ambassador to the Organization for Security and Cooperation in Europe, said that the Kremlin would likely stage “sham” elections in the Russian-backed separatist territories of Donetsk and Luhansk in mid-May to formally seize control of both.

The ambassador would not specify the origin of these reports or how he was able to make such a prediction.

A similar referendum in Kherson, in southern Ukraine, could follow, he said. The Russian language is dominant in all three areas.

“This is straight out of the Kremlin’s playbook,” Mr. Carpenter told reporters at the State Department on Monday.

He said it was not certain that Russia would ultimately move to annex any of the regions, much less be successful in doing so, but that “this is the planning that we are seeing.”

President Vladimir V. Putin of Russia recognized the independence of the so-called Donetsk People’s Republic and Luhansk People’s Republic a few days before Russia’s invasion of Ukraine began in late February. Moscow-backed separatists in the regions have been fighting against Ukrainian forces since 2014.

Mr. Putin annexed the Crimean Peninsula in 2014 just hours after 97 percent of voters in a referendum there approved seceding from Ukraine. The vote was criticized as fraudulent, and much of the world has since refused to recognize Crimea as part of the Russian Federation.

Mr. Carpenter said it was also possible that Russia’s leaders would try to take over other parts of Ukraine, by imposing “puppets and proxies” in local governments and forcing out democratically elected officials. He said that this had appeared to be Moscow’s initial aim in Kyiv — a plan that included installing a new constitution in Ukraine — but that Russian forces had been forced to drop back to the country’s east and south after they were unable to take the capital.

Now, he said, Moscow appears intent on imposing its school curriculum, currency and local leadership in areas where Russian forces are suspected of abducting political opponents, educators and journalists, and cutting off internet services to isolate residents from independent sources of information.

Mr. Carpenter acknowledged there was little that the O.S.C.E. could do to stop Russia, although he cited efforts by the West and other international allies to hammer Moscow with economic sanctions and diplomatic isolation. He said the organization was working to distribute humanitarian relief to Ukrainian people who have been wounded in the war or forced from their homes since Russia invaded, and was helping to document war crimes and other human rights abuses for future prosecutions.

“Part of what we’re trying to do is to expose Russia’s intentions,” Mr. Carpenter said, adding that “unfortunately, we have been more right than wrong in exposing what we believe may be coming next.”