US President Joe Biden speaks during an event. File photo

President Joe Biden says he is now "convinced” Russian President Vladimir Putin has decided to invade Ukraine and assault the capital as more than 40 per cent of the Russian forces on the Ukraine border are now in position for attack and Moscow has begun a campaign of destabilisation, a US defence official said on Friday.

An ominous assessment that emerged as the country's war-torn east saw more attacks that the West said could be designed to establish a pretext for an attack.

After weeks of saying the US was not sure if Putin had made the final decision, Biden said Friday that his judgment had changed, citing American intelligence.

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"As of this moment, I’m convinced he’s made the decision,” Biden said. "We have reason to believe that.” He reiterated that the assault could occur in the "coming days.”

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.

Russian President Vladimir Putin attends a meeting. File photo

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

The US president's comments at the White House followed a day of rising violence that included a humanitarian convoy hit by shelling and a car bombing in the eastern city of Donetsk. Pro-Russian rebels began evacuating civilians from the conflict zone with an announcement that appeared to be part of Moscow’s efforts to paint Ukraine as the aggressor instead.

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.

Meanwhile, the Kremlin announced massive nuclear drills to flex its military muscle, and Putin pledged to protect Russia’s national interests against what it sees as encroaching Western threats.