The White House on Saturday said that Russia could launch an attack against Ukraine at any time and said US President Joe Biden will convene a National Security Council meeting on Sunday to discuss the situation.

Russia's strategic nuclear forces held exercises overseen by President Vladimir Putin on Saturday, and Washington accused Russian troops massed near Ukraine's border of advancing and being “poised to strike.”

A day ago, US President Joe Biden said he was now “convinced” Russian President Vladimir Putin had decided to invade Ukraine and assault the capital, 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 his judgment had changed, citing American intelligence.

“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 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.

As further indication that the Russians are preparing for a major military push, a US defence official said an estimated 40 per cent to 50pc of the ground forces deployed in the vicinity of the Ukrainian border have moved into attack positions closer to the border. That shift has been underway for about a week, other officials have said and does not necessarily mean Putin has decided to begin an invasion.

The defence official spoke on condition of anonymity to discuss internal US military assessments.

The official also said the number of Russian ground units known as battalion tactical groups in the border area had grown to as many as 125, up from 83 two weeks ago. Each group has 750 to 1,000 soldiers.

Russia announced this week that it was pulling back forces from vast military exercises, but US officials said they saw no sign of a pullback and instead observed more troops moving toward the border with Ukraine.

In other developments, the White House and the UK formally blamed Russia for recent cyberattacks targeting Ukraine’s defence ministry and major banks. The announcement was the most pointed attribution of responsibility for the intrusions, which barraged websites with junk data to make them unreachable. Russia rejected the accusations.