US President Joe Biden has labelled Russian leader Vladimir Putin a "war criminal" in a move likely to escalate diplomatic tensions even further.

Mr Biden delivered the remark off-the-cuff in response to a reporter's question at the White House.

It is the first time he has used such language to condemn President Putin, and the White House later said he was "speaking from his heart".

The Kremlin, however, said it was "unforgiveable rhetoric".

"We believe such rhetoric to be unacceptable and unforgivable on the part of the head of a state, whose bombs have killed hundreds of thousands of people around the world," spokesman Dmitry Peskov told Russian state news agency Tass.

The exchange in Washington happened Wednesday when a reporter asked the US president: "Mr President, after everything we have seen, are you ready to call Putin a war criminal?"