A U.S. official is raising concerns that Russian forces could annex the Ukrainian city of Kherson, which is in the southern part of the country near the Crimean Peninsula.

Ambassador Michael Carpenter, who serves as the U.S. diplomat to the Organization for Security and Cooperation in Europe, warned the organization's permanent council on Thursday, "In recent days, these illegitimate proxies have increasingly indicated an intention to 'petition' Russia to annex Kherson."

This is not the first time Carpenter has warned about the possibility of Russia attempting to hold "sham" referendums in Ukrainian cities that Russian forces have seized. At the beginning of May, he told reporters his team had seen "highly credible" evidence to indicate "that the Kremlin may try to hold a sham referendum to try to add a veneer of democratic electoral legitimacy."

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He said that this could happen as soon as "mid-May," though he did not provide a more specific time frame.

Kirill Stremousov, Russia’s depot head of the regional military and civilian administration, said on Wednesday that the Kherson government plans to ask Russian President Vladimir Putin to make the region a part of Russia, according to Russian-state media site TASS.

"The referendum, which was absolutely legally held in Crimea, was not recognized by the world community, which did everything not to recognize Russia as a full-blown member of the global community," he said. "Therefore, this will be one single decree based on the appeal of the leadership of the Kherson region to Russian President Vladimir Putin. There will be a request to make the Kherson region a full-fledged constituent of the Russian Federation."

Earlier this week, Putin gave his much anticipated Victory Day speech, and it was most notable for what he didn't say. There were no declarations of further troop mobilizations. Instead, he focused on repeating his justifications for the war.

While Russia has since refocused its military operation on the Donbas after failing to topple Kyiv, Avril Haines, the U.S. director of national intelligence, told the Senate Armed Services Committee on Tuesday that she's unsure whether Putin would be content if the Russian military succeeded there.

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"Even if they are successful, we are not confident that the fight in the Donbas will effectively end the war," she explained. "We assess President Putin is preparing for a prolonged conflict in Ukraine during which he still intends to achieve goals beyond the Donbas. We assess that Putin's strategic goals are probably not changed, suggesting he regards the decision in late March to refocus Russian forces on the Donbas is only a temporary shift to regain the initiative after the Russian military's failure to capture Kyiv."