US President Joe Biden is to send extra troops to Europe this week amid continuing fears of a Russian invasion of Ukraine, White House officials say.
Some 2,000 troops will be sent from Fort Bragg, North Carolina, to Poland and Germany, and a further 1,000 already in Germany will go to Romania.
Moscow denies planning to invade but has deployed an estimated 100,000 troops near Ukraine's borders.
It fiercely opposes Ukraine joining the US-led Nato military alliance.
The crisis comes eight years after Russia annexed Ukraine's southern Crimea peninsula and backed a bloody rebellion in the eastern Donbas region.
Moscow accuses the Ukrainian government of failing to implement an international deal to restore peace to the east - where Russian-backed rebels control swathes of territory and at least 14,000 people have been killed since 2014.
Ukrainian Foreign Minister Dmytro Kuleba said Russia had not yet amassed enough forces to mount a full-scale invasion and that diplomacy was helping avert the threat of a Russian attack.
The US troops being deployed will not fight in Ukraine but will ensure the defence of US allies.
Of the 2,000 troops being sent from Fort Bragg, 1,700 members of the 82nd Airborne Division will be sent to Poland and the others will go to Germany.
Their deployment is in addition to the 8,500 troops the Pentagon put on alert last month to be ready to deploy to Europe if needed.
Russian President Vladimir Putin is due to speak to UK Prime Minister Boris Johnson by phone on Wednesday. Earlier, on a visit to Ukraine, Mr Johnson accused Russia of putting a "gun to Ukraine's head".
Mr Putin has accused the US of trying to draw his country into a war in Ukraine.
He said America's goal was to use a confrontation as a pretext to impose more sanctions on Russia.
"It seems to me that the United States is not so much concerned about the security of Ukraine... but its main task is to contain Russia's development. In this sense Ukraine itself is just a tool to reach this goal."
Rivalry between Russia and the US, which still possess the world's biggest nuclear arsenals, dates back to the Cold War (1947-89). Ukraine was then a crucial part of the communist Soviet Union, second only to Russia.
Mr Putin said the US had ignored Moscow's concerns in its response to Russian demands for legally binding security guarantees, including a block on the Nato alliance's further expansion to the east.
He suggested that if Ukraine were granted its wish to join Nato, it could drag the other members into a war with Russia.
"Imagine that Ukraine is a Nato member and a military operation [to regain Crimea] begins," the Russian leader said. "What - are we going to fight with Nato? Has anyone thought about this? It seems like they haven't."
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