The United States and its allies will respond with "strong economic and other measures" against Russia if it takes military action against Ukraine, President Joe Biden has said.
In a call with Russian leader Vladimir Putin on Tuesday, Mr Biden "voiced the deep concerns of the United States and our European allies about Russia's escalation of forces surrounding Ukraine", the White House said in a statement.
It said Mr Biden had reiterated his support for Ukraine’s sovereignty and territorial integrity and called for de-escalation and a return to diplomacy. “The two presidents tasked their teams to follow up, and the US will do so in close co-ordination with allies and partners,” the statement said.
It also emerged on Tuesday that a massive $770 billion (€685 billion) defence spending Bill before the US Congress includes $300 million in support for Ukraine's military.
Following his two-hour call with Mr Putin, Mr Biden was scheduled to hold talks with political leaders in France, Germany and the UK regarding the situation in Ukraine.
Mr Biden is expected to speak to Ukrainian president Volodymyr Zelenskiy in the coming days
US intelligence officials say Russia has massed about 70,000 troops near the Ukraine border. The US fears that this move could be a prelude for an invasion of Ukraine, which has had growing links with the West since a pro-Moscow government was ousted in 2014.
Russia denies it has any intention of attacking Ukraine and says its troop movements in the region are routine and defensive.
Written guarantees
US media has reported that the Kremlin is seeking written guarantees from the Americans and its allies that the Nato military alliance will not expand farther east to take in Ukraine as a member or to host western forces.
There have been suggestions in the US that Russia could face financial sanctions if it invades Ukraine, which could see it cut off from global financial markets.
There have also been reports the US is pressing Germany to agree to halt the controversial Nord Stream 2 gas pipeline from Russia in the event of an invasion taking place.
The Reuters news agency said US officials told members of Congress they had an understanding with Germany to shut the Nord Stream 2 pipeline if Russia took such an action.
In a brief snippet from the start of the meeting broadcast by Russia state television, the two leaders offered friendly greetings to each other.
“I welcome you, Mr President,” Mr Putin said, speaking with a Russian flag behind him and a video monitor showing Mr Biden in front of him.
"Good to see you again," Mr Biden replied with a chuckle. He then quickly noted Mr Putin's absence from the recent G20 summit in Rome.
The Russian president took part in the major gathering of industrial nations by video link only because of concerns about Covid-19 at home.
“Unfortunately, last time we didn’t get to see one another at G20,” Mr Biden said. “I hope next time we meet, to do it in person.
The White House also said the US and Russian leaders discussed ransomware, as well as joint work on regional issues such as Iran.
