"It is a war crime to attack a nuclear power plant," tweeted the US embassy in Ukraine -- which was moved out of the capital Kyiv because of the Russian invasion -- after the overnight attack at the nuclear plant in Zaporizhzhia.

Asked by AFP whether Washington was openly accusing Moscow of having perpetrated a war crime prohibited by the Geneva Convention, the US State Department was more cautious.

"The intentional targeting of civilians or civilian objects, including nuclear power plants, is a war crime, and we are assessing the circumstances of this operation," a State Department spokesman said.

"But regardless of the legality, this action was the height of irresponsibility, and the Kremlin must cease operations around nuclear infrastructure," the official said.

Washington has in recent days accused Russia of hitting civilian infrastructure and killing civilians in Ukraine, but has been careful not to explicitly say that the Russian military was intentionally targeting them, or to openly discuss war crimes.

British Prime Minister Boris Johnson on Wednesday called Russian President Vladimir Putin a war criminal.

"What we have seen already from Vladimir Putin's regime, in the use of the munitions that they have already been dropping on innocent civilians, in my view already fully qualifies as a war crime," Johnson said.