By Nandita Bose
WASHINGTON (Reuters) -U.S. President Joe Biden on Monday accused Russian President Vladimir Putin of war crimes and called for a trial, adding to the global outcry over civilian killings in the Ukrainian town of Bucha as more graphic images of their deaths emerged.
"You saw what happened in Bucha," Biden told reporters after landing in Washington from Delaware, where he had spent the weekend. "This warrants him - he is a war criminal."
The discovery of a mass grave and tied bodies shot at close range in Bucha, outside Kyiv, a town Ukrainian forces reclaimed from Russian troops, looked set to galvanize the United States and Europe into imposing additional sanctions against Moscow.
"We have to gather the information. We have to continue to provide Ukraine with the weapons they need to continue the fight. And we have to get all the detail so this can be an actual, have a war crimes trial," Biden said.
The Kremlin categorically denied any accusations related to the murder of civilians, including in Bucha, where it said the graves and corpses had been staged by Ukraine to tarnish Russia.
Biden's National Security Advisor Jake Sullivan told reporters that the United States will seek information from four sources to build a case for war crimes: the U.S. and its allies, including intelligence services; Ukrainian observations on the ground; international organizations including the United Nations; and interviews from global independent media.
He said the United States would build a case at the International Criminal Court or another venue. The United States is not a party to the ICC.
Russia's permanent membership in the United Nations Security Council means any war crimes accountability could be blocked by Moscow in that body, Sullivan said.
Ukrainian President Volodymyr Zelenskiy called the killings "genocide" in a speech from Bucha on Monday as journalists entered the city and documented its destruction.
U.S. defense officials said the Pentagon could not independently confirm the atrocities.
Sullivan said the United States has yet to see evidence that killings reached the level of genocide.
It is not the first time Biden has called Putin a war criminal since he invaded Ukraine on Feb. 24 in what Moscow calls a "special operation." That designation has damaged U.S.-Russian ties, the Kremlin has said.
Citing what he called "horrifying" images, Jim Risch, the leading Republican on the Senate Foreign Relations Committee said in a statement: "The international community must also take concrete steps to hold Putin and his cronies accountable for their war crimes."
Democrat Bob Menendez, committee chairman, said: "Putin must be held accountable for this tragic and barbaric assault on innocent civilians."
Biden's U.N. envoy announced that Washington would also seek Russia's suspension from the U.N. Human Rights Council.