Russia's Vladimir Putin has warned his US counterpart Joe Biden that imposing new sanctions over Ukraine could lead to a complete breakdown in relations.
In a phone call late on Thursday, the Russian president said such sanctions would be a "colossal mistake".
Mr Biden, meanwhile, told Mr Putin that the US and its allies would respond decisively to any invasion of Ukraine.
The call, requested by Russia, was the pair's second such conversation this month and lasted for almost an hour.
It marked the latest effort to defuse tensions over Ukraine's eastern border with Russia, where Ukrainian officials say more than 100,000 Russian troops have been sent.
The build-up has prompted concern in the West, with the US threatening Mr Putin with sanctions "like none he's ever seen" if Ukraine comes under attack.