Russia could launch an attack on Ukraine "at any time," the White House and NATO reaffirmed Saturday, as Western politicians gathered in Munich to discuss the crisis.

US Press Secretary Jen Psaki said President Joe Biden was due to hold a rare Sunday National Security Council meeting over Russia-Ukraine tensions, having said on Friday he was "convinced" Moscow planned to invade its ex-Soviet neighbor within days.

Psaki said Biden was briefed on meetings at the Munich Security Conference, where Western representatives, including Vice President Kamala Harris, gathered to discuss the mounting tensions.

The president's national security team "reaffirmed that Russia could launch an attack against Ukraine at any time," she added.

Western officials in Munich continued to raise the alarm about Moscow's intentions towards Ukraine, though Ukrainian President Volodymyr Zelensky pushed back against Washington's dire predictions for an invasion, saying "we do not think that we need to panic."

The Kremlin insists it has no plans to attack, but Moscow has done little to reduce tensions, with state media accusing Kyiv of plotting an assault on rebel-held, pro-Russia enclaves in eastern Ukraine in the Donetsk and Luhansk regions.

The United States and the European Union have said they believe Moscow is striving to create a pretext for an attack on Ukraine by falsifying violent incidents on the ground and having proxy outlets put out false information.

US State Department spokesman Ned Price on Saturday warned: "We are seeing more evidence of Russian disinformation being used as a pretext for a potential Russian attack on Ukraine. "Locals in Donetsk reported calm despite Russian claims of a car bomb."

NATO chief warning

The signals coming out of Russia suggest that Moscow is readying for a "full-fledged attack" on Ukraine, NATO chief Jens Stoltenberg said Saturday, echoing US warnings of an imminent invasion.

"Every indication indicates that Russia is planning a full-fledged attack against Ukraine," Stoltenberg told German broadcaster ARD on the sidelines of the Munich Security Conference. "We all agree that the risk of an attack is very high."

The EU also warned Saturday it was seeing stepped-up "manipulation" of information to support what looks like a fabricated pretense for military escalation.

The warning came as Russian state media published unverifiable reports of violent acts in eastern Ukraine. 

Ukraine urges West to back 'shield'

Ukrainian President Volodymyr Zelensky said Saturday his country deserved more support in the face of a feared invasion, as NATO chief Jens Stoltenberg warned that everything indicated Russia was preparing a "fully-fledged attack".

As Western politicians gathered in Germany to discuss the crisis, Russia test-fired nuclear-capable missiles in a show of force.

Zelensky condemned "a policy of appeasement" towards Moscow in his speech to the Munich Security Conference.

"For eight years, Ukraine has been a shield," he said. "For eight years, Ukraine has been holding back one of the greatest armies in the world."

Zelensky travelled to Munich despite shelling in his country's conflict-torn east that left two Ukrainian soldiers dead.

Zelensky called for "clear, feasible timeframes" for Ukraine to join the US-led NATO military alliance -- something Moscow has said is a red line for its security.

But the Ukrainian leader also said he was willing to meet with Vladimir Putin, to find out "what the Russian president wants".

Western officials in Munich continued to raise the alarm about Moscow's intentions towards Ukraine, after US President Joe Biden said Friday he was "convinced" Putin planned to invade, including with an attack on the capital Kyiv, within days.

They again warned of enormous sanctions if Russia attacks, with US Vice President Kamala Harris saying this would only see NATO reinforce its "eastern flank".

"Every indication indicates that Russia is planning a full-fledged attack against Ukraine," Stoltenberg told German broadcaster ARD on the sidelines of the conference.

Earlier however, Germany's Foreign Minister Annalena Baerbock had warned against jumping to conclusions.

"In crisis situations, the most inappropriate thing to do is to somehow guess or assume," Baerbock told reporters, after being repeatedly pressed on whether Germany shared Biden's assessment.

"We do not know yet if an attack has been decided on," Baerbock said.

Zelensky also pushed back against Washington's dire predictions in Munich.

"We do not think that we need to panic," Zelensky told the audience of top-level officials and security experts from around the world.

- Strategic missile tests -
The increasingly fraught warnings of an invasion, intense clashes in Ukraine's east and the evacuation of civilians from Russian-backed rebel regions have further heightened fears of a major conflict in Europe after weeks of tensions.

The Kremlin insists it has no plans to attack its neighbour, but Moscow has done little to reduce tensions, with state media accusing Kyiv of plotting an assault on rebel-held pro-Russia enclaves in eastern Ukraine.

Saturday's exercises of strategic forces saw Russia test-fire its latest hypersonic, cruise and nuclear-capable ballistic missiles.

The United States insists that, with around 150,000 Russian troops on Ukraine's borders -- as many as 190,000, when including the Russian-backed separatist forces in the east -- Moscow has already made up its mind to invade.

Around 30,000 Russian troops are in Belarus for an exercise which is due to end on Sunday. Moscow has said these forces will return to barracks, but US intelligence is concerned that they could take part in an invasion of Ukraine.

Russia has in recent days announced a series of withdrawals of its forces from near Ukraine, after what they said were regular military exercises. It has dismissed western claims of an invasion plan as "hysteria".

But Putin has also stepped up his rhetoric, reiterating demands for written guarantees that Ukraine will never be allowed to join NATO and for the alliance to roll back deployments in eastern Europe to positions from decades ago.

- 'Dramatic increase' in clashes -
The volatile frontline between Ukraine's army and separatists in the breakaway regions of Donetsk and Lugansk has seen a "dramatic increase" in ceasefire violations, international monitors from the OSCE European security body have said.

Hundreds of artillery and mortar attacks were reported in recent days, in a conflict that has rumbled on for eight years and claimed the lives of more than 14,000 people.

The OSCE said Saturday there had been 1,500 ceasefire violations in Donetsk and Lugansk in just one day.

Ukraine's army and separatist forces traded accusations of fresh shellfire on Saturday, with Kyiv saying two of its soldiers had died in a shelling attack, the first fatalities in the conflict in more than a month.

A dozen mortar shells fell within a few hundred metres (yards) of Ukraine's Interior Minister Denys Monastyrskiy Saturday as he met journalists on a tour of the frontline.

The rebels declared general mobilisations in the two regions, calling up men to fight even as they announced the mass evacuations of women and children into Russia.

Moscow and the rebels have accused Kyiv of planning an assault to retake the regions, claims fiercely denied by Ukraine and dismissed by the West as part of Russian efforts to manufacture a pretext for war.

Ukrainian Foreign Minister Dmytro Kuleba denounced reports of Ukrainian shells falling on Russian territory as "fake".

Germany and France on Saturday urged their citizens to leave Ukraine. NATO said it was relocating staff from Kyiv to Lviv in the west of the country and to Brussels.

German airlines Lufthansa and Austrian Airlines said they would stop flights to Kyiv and Odessa from Monday until the end of February, but would maintain flights to western Ukraine.