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.

“President Biden said that there is a distinct possibility that the Russians could invade Ukraine in February,” White House National Security Council spokesperson Emily Horne said. “He has said this publicly and we have been warning about this for months. ”

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.

Also read: US offers no concessions in response to Russia on Ukraine

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 nonnegotiable.

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.

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 warned 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.

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.

Also read: Russia sees little optimism in US response on Ukraine crisis

Zelenskyy tweeted that he and Biden also discussed the possibility of additional financial support for Ukraine.

The White House said Biden told Zelenskyy he was “exploring additional macroeconomic support to help Ukraine’s economy” as it comes under pressure as a result of Russia’s military buildup.

Meanwhile, the United States announced that the U.N. Security Council will hold an open meeting Monday on what U.S. Ambassador Linda Thomas-Greenfield called Russia’s “threatening behavior.” She said the deployment of more than 100,000 troops along Ukraine’s border and other destabilizing acts pose “a clear threat to international peace and security and the U.N. Charter.”

Kremlin spokesman Dmitry Peskov told reporters earlier that the response from the U.S. — and a similar one from NATO — left “little ground for optimism.” But he added that “there always are prospects for continuing a dialogue, it’s in the interests of both us and the Americans.”

White House press secretary Jen Psaki was circumspect when asked whether the Biden administration saw a sliver of hope in that the Russians said they would keep communications open even as they said that they lacked optimism..

“We don’t know if the Russians are playing games on diplomacy. We hope not,” Psaki said.

Foreign Minister Sergey Lavrov said the U.S. response contained some elements that could lead to “the start of a serious talk on secondary issues,” but emphasized that “the document contains no positive response on the main issue.” Those are Moscow’s demands that NATO not expand and that the alliance refrain from deploying weapons that might threaten Russia.

Lavrov said top officials will submit proposals to Putin. Peskov said the Russian reaction would come soon.

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.

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.”

On a visit to Denmark, Kuleba emphasized his country’s need to strengthen its defenses.

“This crisis is a moment of truth, and this is why we speak about weapons,” he said. “This is why we speak about economic sanctions. This is why we speak about the consolidated position of all of us, so that President Putin sees that there are no weak links in our defensive chain.”

Germany’s Foreign Minister Annalena Baerbock said during a parliamentary debate on Ukraine that her government is closely coordinating its policy with allies, considering a range of options that could include the new Nord Stream 2 Russian gas pipeline to Germany.

While the diplomacy sputters on, so too do maneuvers that have escalated tensions. Russia has launched a series of military drills involving motorized infantry and artillery units in southwestern Russia, warplanes in Kaliningrad on the Baltic Sea, dozens of warships in the Black Sea and the Arctic, and Russian fighter jets and paratroopers in Belarus.

NATO said it was bolstering its deterrence in the Baltic Sea region, and the U.S. ordered 8,500 troops on higher alert for potential deployment to Europe.

As war fears mounted, thousands of Ukrainians expressed their resolve to stand up to the Russian pressure under the hashtag #UkrainiansWillResist on Twitter and Facebook.

“No one will force Ukrainians to accept the Kremlin ultimatum,” wrote Andrii Levus, who initiated the campaign.

Ukraine’s Interior Ministry has organized training on acting in emergency situations, with an emphasis on dealing with explosives.

Beyond concerns about a possible Russian offensive in Ukraine, there also has been speculation that Moscow’s response could include military deployments to the Western Hemisphere.

While a senior Russian diplomat recently refused to rule out such deployments to Cuba and Venezuela, a top Putin associate expressed skepticism Thursday at that prospect.

“Cuba and Venezuela are aiming to come out of isolation and restore normal relations with the U.S. to a certain extent, so there can’t be any talk about setting up a base there as happened during the Soviet times,” Dmitry Medvedev, a deputy head of Russia’s Security Council, told Russian media.

While he charged that the West is using Ukraine as a way to contain Russia, he somberly acknowledged that a Russia-NATO conflict “would be the most dramatic and simply catastrophic scenario, and I hope it will never happen.”

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.

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.