National leader Christopher Luxon has called Russian President Vladimir Putin a "war criminal", as disturbing reports continue to emerge from the war in Ukraine.

It comes a day after Prime Minister Jacinda Ardern said Russia had to "answer to the world" over reports of Ukrainian civilians being tortured and killed in Bucha and other Kyiv suburbs.

Ardern didn't go as far as labelling Putin a war criminal, preferring to wait for the judgement of the International Criminal Court.

On Tuesday, Luxon said there was a process that needed to be carried out in that court.

"Officially you've got to be prosecuted and convicted before you get labelled a criminal, but I'd have to say to you the evidence is pretty compelling.

"The evidence is there. I would be quite comfortable calling him [Putin] a war criminal."

He said the pictures coming out of Bucha were "alarming".

Luxon also continued to push the Government to expel the Russian ambassador to New Zealand from the country.

"As I said at the beginning of the Ukraine-Russia crisis… we think that sends a really strong signal. We said on day one the time for diplomacy was over, that it was finished and there was no intention for a diplomatic resolution to this."

Ardern said on Monday that she didn't rule out sending the Russian ambassador home. But, she added that doing so would mean New Zealand's ambassador could be removed from Russia.

She said she wanted to keep diplomatic options on the table.

Further sanctions by the New Zealand Government that targeted 36 more Russians with ties to the Putin regime also came into force on Tuesday.

On Monday, images of bodies with bound hands, close-range gunshot wounds and signs of torture were met with condemnation from European leaders.

In the US, President Joe Biden said Putin was "a war criminal".

“You saw what happened in Bucha,” Biden said.

Ukrainian officials said the bodies of 410 civilians were found around the area of Kyiv after the region was retaken by Ukraine from Russian forces.

International Bar Association executive director Dr Mark Ellis told Breakfast on Tuesday morning there was "ample evidence" that war crimes had been committed in Bucha.

UN Secretary-General Antonio Guterres called for an independent investigation into the images from the city.

Russia had denied "violent" action against Bucha residents while it controlled the city - something that contradicted multiple eyewitness accounts, the BBC reported.

International Criminal Court prosecutor Karim Ahmad Khan announced in February that there would be an investigation into what was happening in Ukraine.