The British government has accused Russia of seeking to replace Ukraine's government with a pro-Moscow administration, and said former Ukrainian lawmaker Yevheniy Murayev is being considered as a potential candidate.
Murayev is head of the small pro-Russian party Nashi, which currently has no seats in Ukraine's parliament.
Britain's Foreign Office named several other Ukrainian politicians it said had links with Russian intelligence services.
It is unclear what means Britain believes Russia might use to install a friendly government in Kyiv.
The UK government made the claim based on an intelligence assessment, without providing evidence to back it up. It comes amid a war of words between Moscow and the West over Russia's designs on Ukraine.
Foreign Secretary Liz Truss said the information "shines a light on the extent of Russian activity designed to subvert Ukraine, and is an insight into Kremlin thinking."
Truss urged Russia to "de-escalate, end its campaigns of aggression and disinformation, and pursue a path of diplomacy," and reiterated Britain's view that "any Russian military incursion into Ukraine would be a massive strategic mistake with severe costs."
Britain has sent anti-tank weapons to Ukraine as part of efforts to bolster its defenses against a potential Russian attack.
Amid diplomatic efforts to defuse the crisis, UK Defense Secretary Ben Wallace is expected to meet Russian Defense Minister Sergei Shoigu for talks in Moscow. No timing has been given for the meeting, which would be the first UK-Russia bilateral defense talks since 2013.
The US has mounted an aggressive campaign in recent months to unify its European allies against a new Russian invasion of Ukraine. The White House called the UK government assessment "deeply concerning" and said it stands with the duly elected Ukrainian government.
"This kind of plotting is deeply concerning," said National Security Council spokeswoman Emily Horne. "The Ukrainian people have the sovereign right to determine their own future, and we stand with our democratically-elected partners in Ukraine."
The Baltic nations of Estonia, Latvia and Lithuania plan to send US-made anti-tank and anti-aircraft missiles to Ukraine, a move that the United States fully endorsed Saturday amid Kyiv's escalating tensions with Russia.
The defense ministers of the three Baltic states said in a joint statement that they "stand united in our commitment to Ukraine's sovereignty and territorial integrity in face of continued Russian aggression."
US Secretary of State Antony Blinken said in a tweet that Washington saluted the NATO nations and former Soviet republics "for their longstanding support to Ukraine."
"I expedited and authorized and we fully endorse transfers of defensive equipment @NATO Allies Estonia Latvia Lithuania are providing to Ukraine to strengthen its ability to defend itself against Russia's unprovoked and irresponsible aggression," he said in another tweet.
Kremlin spokesman Dmitry Peskov earlier this week described the West supplying arms to Ukraine as extremely dangerous and said the shipments do nothing to reduce tensions."
Moscow has massed troops near the Russia-Ukraine border, leading to fears of an invasion. The West has rejected Moscow's main demands promises from NATO that Ukraine will never be added as a member, that no alliance weapons will be deployed near Russian borders, and that it will pull back its forces from Central and Eastern Europe.
A meeting on Friday between Blinken and Russian Foreign Minister Sergey Lavrov ended with no breakthrough.
The defense ministers of the Baltic states said that Estonia would provide Javelin anti-tank weapons while Latvia and Lithuania were sending Stinger anti-aircraft missiles and other related equipment to bolster Kyiv's defensive military capabilities.
"Today, Ukraine is at the forefront of separating Europe from the military conflict with Russia. Let's face it, the war in Ukraine is ongoing and it is important to support Ukraine in every way we can so that they can resist the aggressor," Estonian Defense Minister Kalle Laanet said.
Estonia is seeking Germany's approval to send Soviet-made howitzers, which once belonged to East Germany, to Ukraine. Estonia acquired the howitzers from non-NATO member Finland, which in turn had bought them from Germany's military surplus supply in the 1990s.
Germany said Friday it was considering Estonia's request to pass the howitzers on to Ukraine.
Berlin routinely demands a say when German-sold weapons are transferred to third countries. But some recent media reports suggested German Chancellor Olaf Scholz's Cabinet could block Estonia's transfer of weapons to Kyiv.
Ukrainian Foreign Minister Dmytro Kuleba alleged Germany was not showing adequate support for Ukraine.
Also Saturday, the Ukrainian Foreign Ministry summoned the German ambassador to object to recently circulated video in which the head of the German navy said that Ukraine would not regain the Crimean Peninsula, which Russia annexed in 2014, and that Putin deserved "respect".
The comments by vice admiral Kay-Achim Schoenbach sparked consternation and a swift rebuke in Berlin. By late Saturday, the German navy chief had tendered his resignation, saying he wanted to prevent further damage resulting from his "ill-considered statements" in India.
The US State Department is currently warning US citizens not to visit Ukraine due to the pandemic but is also advising them to reconsider travel there due to potential Russian aggression.
