"The only way this ends is for somebody in Russia to take this guy out. You would be doing your country - and the world - a great service."

Fighting rages in key city - Ukraine live updates

'Russiaphobia' in the US is 'off the charts'

The comments prompted a furious response from the Russian ambassador to the United States, Anatoly Antonov, who said the remarks are "unacceptable and outrageous", demanding an explanation.

Mr Antonov said in a statement posted on Facebook: "I find the statement of the American politician unacceptable and outrageous.

"The degree of Russophobia and hatred in the US towards Russia is off the charts. It's unbelievable that a senator of a country that promotes its moral values as a 'guiding star' for all mankind could afford to call for terrorism as a way to achieve Washington's goals in the international arena.

"It's getting scary for the fate of the United States, which has such irresponsible and unprofessional politicians at the helm.

"We demand official explanations and decisive condemnation of this American's criminal statements."

Earlier this week, Sean Hannity, a Fox News host, also called on the assassination of Vladimir Putin.

"You cut the head of the snake off and you kill the snake. Right now the snake is Vladimir Putin," he said.

Attack on Europe's biggest nuclear plant

It comes as Europe's biggest nuclear power plant was shelled by Russian forces in Ukraine.

Three Ukrainian troops were killed and two were wounded in the attack, according to the Ukrainian state nuclear company.

Earlier, Rafael Mariano Grossi, director general of the International Atomic Energy Agency, confirmed no nuclear material had leaked in the air strike.

The attack on the south-eastern city of Energodar and its Zaporizhzhia nuclear plant unfolded as the invasion entered its second week.

A video on the plant's YouTube channel appears to show large explosions at the power station.