Russian attack on Ukraine 'still very much a possibility': Biden

US President Joe Biden said Tuesday that a Russian attack on Ukraine remains "very much" a possibility, but that a diplomatic solution to the crisis could yet be reached.

He said the Russian defense minister reported earlier Tuesday that some military units are leaving their positions near Ukraine.

"That would be good. And we have not yet verified that. We have not yet verified the Russian military units are returning to their home bases. Indeed, our analysts indicate that they remain very much in a threatening position," said Biden.

He also said if Russia attacks Ukraine it would be a "war of choice" or a "war without cause or reason."

"I say these things not to provoke but to speak the truth. If Russia does invade the days and weeks ahead, human costs for Ukraine will be immense," said Biden.

"If Russia attacks Ukraine, it will be met with overwhelming international condemnation. The world will not forget if Russia chose needless death and destruction. Invading Ukraine will prove to be a self-inflicted wound," he added.

Moscow, according to Ukrainian officials and NATO, has recently amassed more than 100,000 troops near Ukraine, prompting fears that the Kremlin could be planning a military offensive against its ex-Soviet neighbor.

Russia has denied it is preparing to invade and accused Western countries of undermining its security through NATO's expansion toward its borders.

Russia also issued a list of security demands to the West, including a rollback of troop deployments from some ex-Soviet states, and guarantees that some of those states would not join NATO.

In a written response to the demands, Washington said it is committed to upholding NATO's "open door policy," while NATO also conveyed the alliance's reply "in parallel with the United States."