France's President Emmanuel Macron discussed the Ukraine crisis with US counterpart Joe Biden on Sunday ahead of a meeting with Russian leader Vladimir Putin, both leaders' offices said.

The two leaders "discussed ongoing diplomatic and deterrence efforts in response to Russia's continued military build-up on Ukraine's borders, and affirmed their support for Ukraine's sovereignty and territorial integrity," the White House said in a statement.

The 40-minute phone call was part of coordination efforts, the French presidency said, before Macron travels to Moscow on Monday and on to Kyiv on Tuesday, where he is due to meet Ukrainian President Volodymyr Zelensky.

The French leader has indicated that he is going to "discuss terms of a de-escalation" of the crisis.

Over the weekend, Macron also held talks with British Prime Minister Boris Johnson and NATO chief Jens Stoltenberg plus the leaders of Baltic nations.

Macron and Biden had spoken earlier in the week, pledging to coordinate their response to Russia's military buildup on the Ukrainian border and reaffirming their support for Ukraine's sovereignty and territorial integrity.

President Macron, talking to the JDD weekly, said ""we have to be very realistic," cautioning that "we will not obtain unilateral gestures" from Russia.

He added that it is "essential to avoid a deterioration of the situation before building mechanisms and reciprocal gestures of trust".

Diplomatic solution 'more likely' than Ukraine invasion

Ukraine's presidency has insisted the chance of resolving soaring tensions with Russia through diplomacy remains greater than that of an attack, as the US warned Moscow was stepping up preparations for an invasion.

"An honest assessment of the situation suggests that the chance of finding a diplomatic solution for de-escalation is still substantially higher than the threat of further escalation," said presidency advisor Mykhailo Podolyak in a statement.

Earlier, US officials said Russia had stepped up preparations for an all-out invasion of Ukraine but that it is not clear if Moscow has decided to take such a step, citing intelligence assessments.

Russia has assembled 110,000 troops along the border with its pro-Western neighbour, but US intelligence has not determined if President Vladimir Putin has actually decided to invade, according to the officials who in recent days briefed members of Congress and European allies.

The officials warned that the assembled Russian force on the frontier is growing at a rate that would give Mr Putin the force he needs for a full-scale invasion - some 150,000 soldiers - by mid-February.

They said Mr Putin wants all possible options at his disposal: from a limited campaign in the pro-Russian Donbas region of Ukraine, to a full-scale invasion.

Russia denies that it is planning to invade Ukraine.

Meanwhile, Chancellor Olaf Scholz said on Sunday that Germany was prepared to send extra troops to the Baltic states, ahead of a key trip to Washington where he will seek to bolster his influence in the Ukraine crisis.

"We are... prepared to do whatever is necessary to strengthen" Germany's presence in NATO operations in the Baltics, Scholz said in an interview with the ARD broadcaster.

Germany leads a NATO operation in Lithuania and has around 500 soldiers stationed there.

Asked whether reinforcements could be agreed at a NATO defence ministers' meeting in mid-February, Scholz said: "We are ready to make a decision."

Ukrainian servicemen take part in tactical exercises in the ghost city of Pripyat, near Chernobyl

If Moscow does opt for a full-scale attack, the invading force could take the capital Kyiv and topple President Volodymyr Zelensky in a matter of 48 hours, the officials said.

They estimated such an attack would leave 25,000 to 50,000 civilians dead, along with 5,000 to 25,000 Ukrainian soldiers and 3,000 to 10,000 Russian ones.

It could also trigger a refugee flood of one to five million people, mainly into Poland, the officials added.

US President Joe Biden has decided to send American forces to Poland to protect NATO members, as diplomats work furiously to try to persuade Russia to pull its troops back from the border with Ukraine.

The first contingent of US soldiers arrived yesterday.

Russia has also announced what it calls joint military manoeuvers with Belarus, where it has sent several battalions to the north of Kyiv and in the Brest region, not far from the border with Poland.

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.

Civilians participate in a Territorial Defence unit training session in Obukhiv, Ukraine yesterday

But on Friday, there were 80 battalions and 14 more were en route from elsewhere in Russia, US officials said.

They added that some 1,500 Russian special forces soldiers known as Spetsnaz arrived along the Ukraine border a week ago.

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 US officials said.

They added that another six amphibious craft were observed leaving the Barents Sea north of Russia, sailing past Britain and through the Strait of Gibraltar, 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, US officials said.

On Thursday, the United States said it had evidence that Russia was preparing a video depicting a bogus attack by Ukraine that would serve as a pretext for a real Russian assault on Ukraine.