Given all of that potential evidence in the public domain, Khan earlier this week said there was a “reasonable basis to believe” that war crimes and crimes against humanity have been committed in Ukraine.

Russian troops have killed hundreds of civilians, including an unknown number of children, and shelled apartment buildings and neighbourhoods in their assault on Ukraine. Ukraine said more than 2,000 civilians have died, a claim that could not be independently verified.

Zelensky described Putin’s attack on the residential centre of Ukraine’s second-largest city Kharkiv as “frank, undisguised terror. Nobody will forgive. Nobody will forget. This attack on Kharkiv is a war crime”.

Russian forces capture Ukraine city of Kherson, a strategic gain for Moscow

The number of people sent fleeing Ukraine by Russia’s invasion has also topped 1 million, the swiftest refugee exodus this century.

The tally from the UN refugee agency released to Associated Press amounts to more than 2 per cent of Ukraine’s population being forced out of the country in less than a week.

A mass evacuation could be seen in Kharkiv, where residents desperate to get away from falling shells and bombs crowded the city’s train station and tried to press onto trains, not always knowing where they were headed.

Ukraine is not a party to the Rome Statute of the ICC but the country has accepted the court’s jurisdiction to try war crimes and crimes against humanity on its territory, according to the prosecutor. Russia does not recognise the court.

US fears for civilians as Ukraine toll grows under Russian assault

The court had already investigated incidents related to the crackdown on pro-European protesters in Kyiv in 2013-2014, as well as the Russian occupation of Crimea and eastern Ukraine in 2014.

Anthony Dworkin, a war crimes expert and senior policy fellow at the European Council on Foreign Relations in London, said Putin’s actions in Ukraine follow a consistent pattern dating back to the military conquest of Chechnya in 1999 that preceded his rise to power and Russia’s actions more recently in Syria.

“There’s a lot of strong evidence of Russian forces attacking civilian infrastructure, attacking hospitals, basically attacking civilian buildings,” Dworkin said. “All of those are war crimes.”

“But I don’t think he cares. It doesn’t matter,” said David Schwendiman, a former senior Justice Department lawyer and international war crimes prosecutor.

Schwendiman noted that Putin’s armies have committed numerous war crimes before. “This is a pure and simple exercise of power. And the law in these circumstances will probably never constrain that kind of exercise of power.”

Ukraine crisis: what previous wars tell us about Putin’s next moves

Another war crimes expert, former Pentagon special counsel Ryan Goodman, was sceptical about whether the ICC – and the related International Court of Justice – could make an immediate difference.

“The wheels of international criminal justice turn slowly, so it is unclear how much of an impact (the two international courts) can have in the present crisis,” said Goodman, a member of the State Department’s Advisory Committee on International Law. “The Russians are considering risks and benefits on a much shorter time horizon.”

“We are following it very closely,” Biden said as he departed the White House. “It’s too early to say that.”

Tribune News Service, Agence France-Presse and dpa