Russia's leader will oversee major military drills along Ukraine's borders on Saturday, further escalating tensions after Washington said Moscow would invade within days, and Ukraine's president headed to Europe to drum up support.

Artillery shelling in the east of Ukraine and orders from Russian-backed separatists for civilians to evacuate the region Friday inflamed an already febrile situation as Washington insisted Moscow was encircling its pro-Western neighbour.

The Kremlin continues to say it has no plans to attack.

US President Joe Biden said that the invasion would come in the next week or days and that his Russian counterpart Vladimir Putin had "made the decision" to invade. But Biden left the door open for a diplomatic resolution.

"Russia has a choice between war and all the suffering it will bring or diplomacy that will make a future for everyone," Biden said at the White House Friday.

Ukrainian President Volodymyr Zelensky was due to travel to Germany Saturday to meet Western leaders, with talks between him and US Vice President Kamala Harris expected.

Biden said it "may not" be wise for Ukraine's leader to leave his country as war fears reached a fever pitch.

The United States says that with an estimated 149,000 Russian troops on Ukraine's borders -- as many as 190,000, when including the Russian-backed separatist forces -- an attack is inevitable.

The Russians have never given a figure for the deployment along the border with Ukraine nor how many are taking part in ongoing drills with neighbouring Belarus.

Compounding fears, Russia's defence ministry announced that President Vladimir Putin would personally oversee previously scheduled drills involving nuclear-capable missiles on Saturday.

- 'Change the dynamics' -

"He's focussed on trying to convince the world he has the ability to change the dynamics in Europe in a way that he cannot," Bided added.

There were growing fears that a spark, which Washington warns could be a deliberate "false flag" incident orchestrated by Moscow, could set off the largest military confrontation in Europe since World War II.

NATO chief Jens Stoltenberg, attending the Munich Security Conference, warned the size of the assembled Russian force far exceeded that needed for military drills, and that Russia had the capacity to invade without warning.

France and Germany have urged Russia to use its influence on rebels in Ukraine's disputed east to "encourage restraint and contribute to de-escalation".

But on the ground, a spike in clashes has fed a growing sense of dread.

An AFP reporter near the front between Ukrainian government forces and pro-Russian separatists in the Lugansk region heard explosions and saw damaged civilian buildings on Kyiv's side of the line.

Monitors from the Organization for Security and Co-operation in Europe said Saturday they had seen a significant rise in the number of attacks along the front line, particularly in the separatist areas of Donetsk and Lugansk.

Attempting to reverse the aggressor narrative, Moscow-backed leaders have accused Kyiv of planning an offensive to retake the eastern territories. The evacuations of civilians there were said to be in response to worries about a government attack.

There were also reports in Russian media that an oil pipeline had exploded in Lugansk.

But US Secretary of State Antony Blinken, who will meet his Russian counterpart for talks Thursday according to Biden, accused the Kremlin of mounting a propaganda campaign to create an excuse for war.

Biden again ruled out sending US troops into Ukraine, but his administration reiterated that it would hit Moscow with costly sanctions that would transform Russia into "a pariah to the international community".

US says 40% of Russian forces on Ukraine border in attack position

More than 40 percent of the Russian forces on the Ukraine border are now in position for attack and Moscow has begun a campaign of destabilization, a US defense official said Friday.

The United States, which estimates that Russia has placed more than 150,000 troops near Ukraine's borders, has observed significant movements since Wednesday, the official said, insisting on anonymity.

"Forty to fifty percent are in an attack position. They have uncoiled in tactical assembly in the last 48 hours," the official told reporters.

Tactical assembly points are areas next to the border where military units are set up in advance of an attack.

The official said Moscow had massed 125 battalion tactical groups close to the Ukraine border, compared to 60 in normal times and up from 80 at the beginning of February.

The increase in clashes between pro-Russian separatists and Ukraine government forces in the southeastern Donbas region of Ukraine, and inflammatory claims by officials in Russia and Donbas, show that "the destabilization campaign has begun," the official said.

Washington has warned for weeks that Russia could provoke or fabricate an incident in the area to serve as a pretext for invading Ukraine.

US Secretary of Defense Lloyd Austin told ABC News' "This Week" that Russian President Vladimir Putin "has a number of options available to him and he could attack in short order."

"I don't believe it's a bluff," Austin said, adding, "I think he's assembled... the kinds of things that you would need to conduct a successful invasion."

Moscow denies it has plans to attack its western neighbor, but is demanding a guarantee that Ukraine will never join NATO and that the Western alliance remove forces from Eastern Europe, demands the West has refused.

In 2014, Russia invaded and occupied the Crimea region of Ukraine, making use of sympathetic separatists.