"I'll be moving troops to Eastern Europe and the NATO countries in the near term. Not too many," Biden told reporters on return to Washington from a speech in Philadelphia.

The United States already has tens of thousands of troops stationed across mostly Western Europe, but the Pentagon is discussing sending a small number of reinforcements to the tense eastern flank.

This week, spokesman John Kirby said 8,500 troops were on "heightened alert" for possible deployment to assist NATO.

The deployment would be as much politically as militarily significant, bolstering US involvement in the brewing conflict.

Ukraine is not a member of NATO, but Washington fears spillover into neighboring NATO countries if Russia attacks Ukraine.

Moscow insists it does not plan to attack but has stationed more than 100,000 combat troops on the borders and is demanding that Western powers exclude Ukraine from ever joining the NATO alliance, as well as other concessions.

Biden warned in a press conference last week that a Russian assault on Ukraine would achieve the opposite to the Kremlin's stated goal.

"We're going to actually increase troop presence in Poland, in Romania, etcetera if in fact he moves," Biden said. "They are part of NATO."