• President Joe Biden said evidence needs to be gathered to put Russian leader Vladimir Putin on trial for war crimes related to his nation's invasion of Ukraine.
• "He is a war criminal," Biden said of Putin, on the heels of reports of mass killings of civilians by Russian troops in the town of Bucha, northwest of Ukraine's capital of Kyiv.
• Biden also said he plans to seek additional sanctions on Russia for its conduct during the war.

President Joe Biden on Monday called for evidence to be gathered to put Russian leader Vladimir Putin on trial for war crimes related to his nation's invasion of Ukraine.

"He is a war criminal," Biden said of Putin, on the heels of reports of mass killings of civilians by Russian-controlled troops in the town of Bucha, northwest of Ukraine's capital of Kyiv.

"This guy is brutal, and what's happening in Bucha is outrageous and everyone's seen it," Biden told reporters, a day after video and still images revealed the town's streets littered with dead bodies.

"I think it is a war crime. ... He should be held accountable."

Biden also said he plans to slap additional sanctions on Russia for its conduct during the war, which began with an invasion on Feb. 24.

"I'll let you know," the president replied when a reporter asked him the nature of those expected sanctions.

National security advisor Jake Sullivan later Monday told reporters at the White House that sanctions would be announced this week.

"Russia will pay a severe price," Sullivan said. "We have communicated to them directly. We have coordinated with our allies and partners and I'm not going to go for the specifics here."

Ukrainian President Volodymyr Zelenskyy earlier accused Russian forces of committing genocide, saying that Ukrainians are being "destroyed and exterminated."

Bucha's mayor has said about 300 residents of the town had been killed at the time that fighters from Chechnya controlled the area.

Russia has denied allegations that its troops killed civilians in Bucha, with its Defense Ministry calling the claims a "provocation."

Biden said Monday, "You may remember, I got criticized for calling Putin a war criminal" last month.

"Well, the truth of the matter, you saw what happened in Bucha. This warrants him — he is a war criminal," Biden said.

"But we have to gather the information. We have to continue to provide Ukraine with the weapons they need to continue the fight," the president said.

"We have to get all the detail so this can be ... a war crime trial."

The International Criminal Court's chief prosecutor has opened an investigation into Russia's conduct in Ukraine.

Sullivan, the national security advisor, at the White House briefing when asked about the forum for a war crimes trial of Putin said, "We have to sit with our allies and partners on what makes most sense as a mechanism moving forward,"

"Obviously, the ICC is one venue where war crimes have been tried in the past, but there have been other examples," Sullivan said. "So there's work to be done to work out the specifics of that."

— Additional reporting by Amanda Macias.