(WASHINGTON) — President Joe Biden met with his top national security advisers Monday as U.S. officials continued to warn a Russian invasion of Ukraine appeared imminent — while diplomats considered a possible summit between Biden and Russian President Vladimir Putin.

Vice President Kamala Harris, Defense Secretary Lloyd Austin, CIA Director William Burns, and the chairman of the Joint Chiefs of Staff, Gen. Mark Milley, all arrived at the White House on Presidents Day morning.

In Moscow, Putin announced during a Monday meeting with his own national security team that he would decide by the end of the day if he would recognize two breakaway Ukrainian regions as independent, a move that analysts believe could be a precursor to Russia annexing them and possibly sending in troops. The Kremlin said he would deliver an address to Russians late Monday.

While the U.S. and Western allies have said they would be united in imposing severe sanctions on Russia if it invades Ukraine, they have been more ambiguous about what steps they would take if Russia stopped short of a full-on invasion. Secretary of State Antony Blinken said last week that Russia recognizing the regions’ independence would “necessitate a swift and firm response from the United States in full coordination with our allies and partners.”

The meetings came a day after the White House said Biden was, “in principle,” open to a summit with Putin, brokered by France’s President Emmanuel Macron, on the condition that Russia did not invade. Russian officials were cool to the idea on Monday.

During their meeting with Putin, Russia’s defense minister, foreign minister, chiefs of intelligence agencies, and the heads of parliament and senate, all called on Putin to recognize the Ukrainian regions of Donetsk and Luhansk, currently controlled by pro-Russian separatists.

Biden said Friday the U.S. had “reason to believe” that Russia would invade “within days.” On Sunday, U.S. officials told ABC News that lower-level Russian tactical commanders had been making plans on the ground, at the local level, to invade Ukraine.

A senior Biden administration official said Sunday that no plans existed yet for a potential Biden-Putin summit, and that Secretary of State Antony Blinken and Russian Foreign Minister Sergey Lavrov would discuss the format and timing later this week — as long as Russia did not invade.

The diplomatic proposal emerged from two calls Macron held with Putin and one with Biden Sunday; his second with Putin began around 1 a.m. Moscow time Monday morning, according to the Elysée Palace.

Biden told Macron that, “in principle, he would be prepared to meet with Putin if President Putin stood down from his invasion,” Biden’s top national security adviser, Jake Sullivan, said in an interview with ABC News’ Good Morning America on Monday.

But, Sullivan added, “We can’t say anything other than indications on the ground look like Russia is still moving forward.”

Meanwhile, during their meeting in Moscow, Putin and top Russian national security officials bluntly questioned the usefulness of holding any new summit with Biden, suggesting it would be pointless unless the United States had changed its position.

Putin said that Macron suggested there were some “changes” in the U.S. position, although he added he could not see what they would be. Russia’s foreign minister said he would speak to his French counterpart on Monday — but was sure the U.S. would not provide positive responses to Russia’s needs.

Even as U.S. officials warned a Russian invasion appears imminent, they also said they were still open to talking.

“We never give up hope on diplomacy until the missiles fly or the tanks roll,” Sullivan said. “We’ve been working hard for months with our allies and partners to get Russia to sit down in a serious way at the table.”

But, he added, “The likelihood there’s a diplomatic solution, given the troop movements of the Russians, is diminishing hour by hour.”

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