A Russian attack on Ukraine is "still very much a possibility" and the human cost would be "immense", US President Joe Biden has said, BBC reported.

In remarks televised nationally, he said the US was ready to respond decisively to such a move.

The US president said Russia had now massed some 150,000 troops on the border with Ukraine.

Russia's defence minister has said some forces have withdrawn. Mr Biden said that this had not been verified.

"[Russian forces leaving] would be good, but we have not yet verified that. We have not yet verified the Russian military units are returning to their home bases," the US president said.

"Indeed, our analysts indicate that they remain very much in a threatening position."

On Tuesday, Ukraine said the websites of its Defence Ministry and two banks came under a cyber-attack. The cause is not clear but Ukraine has suffered large-scale attacks before on its online infrastructure and has pointed the finger at Russia.

There has long been concern that, rather than a full-scale invasion, Russia could use less obvious means to destabilise Ukraine, for example through cyber-attacks on critical infrastructure. President Biden said on Tuesday the US was prepared to respond to any such move.

Mr Biden's speech came hours after Russian President Vladimir Putin said Moscow's security concerns should be addressed and taken seriously.

Mr Putin has always denied planning an incursion, and said Russia did not want another war in Europe. However, tensions have been rising since November.

Following earlier reports that US intelligence believed Russia could attack Ukraine on Wednesday, Ukrainian President Volodymyr Zelensky declared it to be a day of unity, calling on citizens to wave the country's flag and wear blue and yellow ribbons - Ukraine's national colours.

More about: