The US president, who has spent weeks trying to lead a united trans-Atlantic response to Russia's aggression against its neighbor, was meeting "with fellow NATO heads of state and government in an extraordinary virtual summit to discuss the security situation in and around Ukraine," the White House said.

The meeting, with Biden joining from the White House Situation Room, was not open to the media.

It came as Russian troops entered parts of the capital Kyiv after assaulting Ukraine from multiple directions and Russian President Vladimir Putin called for the ouster of the country's government.

Caught between wanting to resist Russia's blatant overturning of post-World War II European security norms and unwillingness to risk confrontation between the nuclear armed powers, NATO is walking a fine line in the face of what looks like a resurgence of the Cold War.

Ukraine is not part of NATO, but four neighboring countries are and the United States has rushed troops to the eastern flank to reassure allies jittery about Putin's broader intentions.

NATO also finds itself enmeshed in the conflict because it was Ukraine's longterm ambition of joining the alliance and the European Union -- in hopes of fully breaking free from Russian domination -- that in part prompted the Kremlin's decision to attack.

Biden has repeatedly said that Ukraine is nowhere near being able to join NATO and is also firm that US troops will not go there to help push back Russia.

Despite that, the United States and other members of the preeminent Western alliance have been heavily involved in trying to beef up Ukraine's beleaguered military with weapons shipments, while border countries like Poland are bracing for huge flows of Ukrainian refugees.

Addressing Americans in a White House speech Thursday, Biden was firm about the US commitment to defending Europe.

"As I made crystal clear, the United States will defend every inch of NATO territory with a full force of American power," Biden said.

The president added, however: "Our forces will not be engaged in a conflict with Russia in Ukraine."

On Friday, Recep Tayyip Erdogan, president of NATO member Turkey, accused the alliance and the European Union of failing to take a "determined stance" on Ukraine.

"NATO should have taken a more decisive step," he said, expressing hope that Friday's summit would not be simply a session of "advice and condemnation," but lead to a "more determined approach."

With fears that Putin might have designs beyond Ukraine -- for example testing NATO's readiness to risk conflict over vulnerable Baltic states -- the most likely next steps are for even more alliance troops to be sent to the east.

A diplomat told AFP that "extra defensive land and air forces are going to be deployed to the alliance's eastern flank, and there will be added naval measures."