The White House says President Joe Biden warned Ukraine’s president Thursday that there is a “distinct possibility” Russia could take military action against Ukraine in February. The Kremlin likewise sounded a grim note, saying it saw “little ground for optimism” in resolving the crisis after the U.S. this week again rejected Russia’s main demands.

Russian officials said dialogue was still possible to end the crisis, but Biden again offered a stark warning amid growing concerns that Russian President Vladimir Putin will give the go-ahead for a further invasion of Ukrainian territory in the not-so-distant future.

The White House said Biden’s comments to Ukraine’s Volodymyr Zelenskyy in a phone call amplified concerns that administration officials have been making for some time.

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Tensions have soared in recent weeks, as the United States and its NATO allies expressed concern that a buildup of about 100,000 Russian troops near Ukraine signaled that Moscow planned to invade its ex-Soviet neighbor. Russia denies having any such designs — and has laid out a series of demands it says will improve security in Europe.

But as expected, the U.S. and the Western alliance firmly rejected any concessions on Moscow’s main points Wednesday, refusing to permanently ban Ukraine from joining NATO and saying allied deployments of troops and military equipment in Eastern Europe are non negotiable.

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The U.S. did outline areas in which some of Russia’s concerns might be addressed, possibly offering a path to de-escalation. But, as it has done repeatedly for the past several weeks, Washington also warned Moscow of devastating sanctions if it invades Ukraine. In addition to penalties targeting Russian people and key economic sectors, several senior U.S. officials said Thursday with certainty that Germany would not allow a newly constructed gas pipeline to begin operations in the event of an incursion.

All eyes are now on Putin, who will decide how Russia will respond amid fears that Europe could again be plunged into war.

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In the meantime, Biden spoke to his Ukrainian counterpart Zelenskyy on Thursday to reiterate American and allied support, including recent deliveries of U.S. military aid.

Biden warns Zelenskyy that the U.S. believed there was a high degree of likelihood that Russia could invade when the ground freezes and Russian forces could attack Ukrainian territory from north of Kyiv, according to two people familiar with the conversation who were not authorized to comment publicly.

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Military experts have said Russia may be waiting for optimal ground conditions to move heavy equipment into Kyiv as part of any invasion. Eight years ago, Russia invaded Ukraine’s Crimean Peninsula in late February.

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The evasive official comments reflect the fact that it is Putin who will single-handedly determine Russia’s next moves. He has warned of unspecified “military-technical measures” if the West refuses to heed the demands.

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Peskov added that Putin and Biden will decide whether they need to have another conversation following two calls last month.

Ukraine’s Foreign Minister Dmytro Kuleba said Kyiv had seen the U.S. response before it was delivered to Russia and had no objections. He tweeted it was “important that the U.S. remains in close contact with Ukraine before and after all contacts with Russia.”

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While concerns about a possible Russian attack linger, a separatist conflict simmers in Ukraine. Following the 2014 ouster of a Kremlin-friendly president in Kyiv, Moscow annexed Ukraine’s Crimean Peninsula and backed an insurgency in the country’s eastern industrial heartland. Fighting between Ukrainian forces and Russia-backed rebels has killed over 14,000 people, and efforts to reach a settlement have stalled.

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Since the conflict began, Russia has been accused of sending troops and weapons to the separatists, something it has denied. On Thursday, Peskov wouldn’t comment on a proposal from the Kremlin’s main political party, United Russia, which suggested that Moscow respond to the delivery of Western weapons to Ukraine by sending arms to the rebels. He added that Putin is aware of the proposal but had no immediate reaction.