With war clouds gathering over Ukraine, international diplomacy went into overdrive on Monday with the French and Russian presidents talking in Moscow and Germany's chancellor heading to the White House to meet with United States leader Joe Biden as Kyiv downplayed a possible military conflict with Russia, saying do not to believe "apocalyptic predictions."

Also on Monday, the German, Czech, Slovak and Austrian foreign ministers were expected in Kyiv, which has played down dire U.S. warnings that Moscow had stepped up preparations for a major incursion into Ukraine.

U.S. officials have said the Kremlin has assembled 110,000 troops along the border with its pro-Western neighbor, but intelligence assessments have not determined whether Russian President Vladimir Putin has actually decided to invade.

They said Russia is on track to amass a large enough force – some 150,000 soldiers – for a full-scale invasion by mid-February.

Such a force would be capable of taking the capital Kyiv in a matter of 48 hours in an onslaught that would kill up to 50,000 civilians, 25,000 Ukrainian soldiers and 10,000 Russian troops and trigger a refugee flood of up to 5 million people, mainly into Poland, the officials added.

On top of the potential human cost, Ukraine fears further damage to its already struggling economy.

And if Moscow attacks Ukraine it could face retaliation over the Nord Stream 2 pipeline – set to double natural gas supplies from Russia to Germany – with Berlin threatening to block it.

Russia is seeking a guarantee from NATO that Ukraine will not enter the alliance and wants the bloc to withdraw forces from member states in Eastern Europe.

'Apocalyptic predictions'

Moscow denies that it is planning to invade Ukraine, and Kyiv's presidency advisor said the chances of a diplomatic solution to the crisis remained "substantially higher than the threat of further escalation."

On Twitter, Ukraine's Foreign Minister Dmytro Kuleba sought to calm tensions.

"Do not believe the apocalyptic predictions. Different capitals have different scenarios, but Ukraine is ready for any development," Kuleba wrote on Twitter. "Today, Ukraine has a strong army, unprecedented international support and the faith of Ukrainians in their country. It is the enemy who should fear us."

A day earlier two U.S. officials said that Russia may be ready for a full-scale invasion of Ukraine by mid-February as it had in place about 70% of the combat power it believed it would need and was sending more battalion tactical groups to the border.

White House national security adviser Jake Sullivan said on Sunday Russia could take military action "any day now" but could still opt for diplomacy.

Ukrainian President Volodymyr Zelenskyy said a week ago Ukraine was not a sinking Titanic and accused Washington and media of fueling panic that weighed on the economy when there were "no tanks in the streets." Presidency advisor Mykhailo Podolyak insisted that the chances of finding a diplomatic solution to the crisis remained "substantially higher than the threat of further escalation."

French President Emmanuel Macron, which currently holds the rotating presidency of the EU, will be in Moscow on Monday and Kyiv on Tuesday to spearhead efforts to de-escalate the crisis. He is expected to push forward a stalled peace plan for the festering conflict with Russian-backed separatists in eastern Ukraine.

The trip will be a political gamble for Macron, who faces a reelection challenge in April.

Also on Monday, German Chancellor Olaf Scholz will meet with Biden in Washington.

"We worked hard to send a clear message to Russia that it will have a high price if they were to intervene into Ukraine," Scholz told The Washington Post in an interview ahead of his meeting. "I really appreciate what President Biden is doing in the bilateral talks between the United States and Russia. They are very difficult."

Biden has reacted to the Russian troop buildup by offering 3,000 American forces to bolster NATO's eastern flank, with a batch of the troops promised arriving in Poland on Sunday. But U.S. national security advisor Jake Sullivan told Fox News Sunday that Biden "is not sending forces to start a war or fight a war with Russia in Ukraine."

"We have sent forces to Europe to defend NATO territory," he said.

Scholz said Sunday that Berlin was prepared to send extra troops to the Baltics in addition to the 500 soldiers already stationed in Lithuania under a NATO operation. While he is in Washington, his foreign minister, Annalena Baerbock, will be in Kyiv along with her Czech, Slovak and Austrian counterparts for a two-day visit. Scholtz will be in Moscow and Kyiv next week for talks with Putin and Zelenskyy.

Special forces, naval buildup

U.S. intelligence has concluded that Russia is continuing to muster a major military force on its border with Ukraine.

Two weeks ago, a total of 60 Russian army battalions were positioned to the north, east and south of Ukraine, particularly in the Crimean Peninsula, which Russia annexed after an invasion in 2014.

By Friday, there were 80 battalions and 14 more were en route from elsewhere in Russia, U.S. officials said. They added that some 1,500 Russian special forces soldiers known as Spetsnaz arrived along the Ukraine border a week ago.

Russia has also announced what it calls joint military maneuvers with Belarus, where it has sent several battalions to the north of Kyiv.

A major Russian naval force is also positioned in the Black Sea, equipped with five amphibious vessels that could be used to land troops on Ukraine's southern coast, the U.S. officials said.

They added that another six amphibious craft were observed leaving apparently on their way to the Black Sea.

In other deployments, Russia has positioned fighter planes near Ukraine, as well as bombers, missile batteries and anti-aircraft batteries, U.S. officials said.