Joe Biden has warned that he will not accept "red lines" set by Moscow as fears mount that Russia is planning an imminent invasion of Ukraine.
The US president said he will make it "very, very difficult" for Russia to invade its neighbour.
Meanwhile, US media has reported that intelligence officials fear an invasion could begin in early 2022.
It comes as Ukraine says Russia has boosted its military at the border and amassed some 94,000 troops there.
A video call between Russian President Vladimir Putin and Mr Biden to ease tensions is expected this week. The US President told reporters he is expecting to have a long discussion with the Russian leader, and warned that he will not "accept anybody's red lines".