WASHINGTON, Dec 29 - President Biden is expected to speak by phone to Russian President Vladimir Putin on Thursday — at the Kremlin's request - weeks after the White House said Biden made it clear to Putin that if Russia decided to invade Ukraine, the United States and its European allies stand ready to respond with economic sanctions and military support, NPR reported.

Speaking with reporters on Wednesday, a senior administration official said the White House did not know why Russia had called for a conversation. But the call comes as the U.S. continues its diplomatic efforts over Russia's military buildup near its border with Ukraine.

"I think both leaders believe that there is genuine value in direct leader-to-leader engagement, that we are at a moment of crisis and have been for several weeks now given the Russian buildup and that it will take a high level of engagement to address this and to try to find a path to de-escalation," the administration official said.

President Volodymyr Zelensky and U.S. Secretary of State Antony Blinken held talks on the settlement in Donbas and contacts with Russia on Wednesday, the press service of the U.S. Department of State said.

"Secretary Blinken reiterated the United States' unwavering support for Ukraine's independence, sovereignty, and territorial integrity in the face of Russia's military buildup on Ukraine's borders. The two discussed efforts to peacefully resolve the conflict in eastern Ukraine and upcoming diplomatic engagements with Russia," Spokesperson Ned Price said.

Tens of thousands of Russian troops have been deployed to the border with Ukraine, prompting fears that Russia is preparing to launch a military invasion — as it did in 2014, during Biden's vice presidency, when it annexed Ukraine's Crimean Peninsula.

Since the two leaders spoke earlier this month, there have been no signs of de-escalation around Ukraine, and the White House remains concerned about Russia's increased presence at the border.

"I would simply say that we continue to be gravely concerned about the nature of the Russian troop presence there — the capabilities that they have," the senior administration official said. "It is not entirely static from our perspective. I won't go into more detail on this line, but it remains a continuing source of grave concern what the Russians have been putting in place in and around that area." (om/ez)