US President Joe Biden on Monday called for his Russian counterpart to face a war crimes trial after apparent evidence of mass atrocities by Russian forces emerged from the Ukrainian city of Bucha, near the capital Kyiv.

Biden called what is being uncovered in Bucha, a city Russian troops recently withdrew from, "outrageous."

President Vladimir Putin "is a war criminal" who "should be held accountable," Biden told reporters. He continued to maintain that the US and international community must gather additional evidence to support war crimes allegations ahead of any legal proceeding.

"This guy is brutal," Biden said of Putin. "What's happening in Bucha is outrageous, and everyone's seen it."

Biden said he believes what he has seen falls short of constituting genocide, but is nonetheless a war crime. He further vowed to impose additional sanctions on the Kremlin.

The bodies of at least 410 civilians were recovered in the city of Bucha on Sunday, according to Ukraine's prosecutor-general. Russia has denied that its troops killed civilians while withdrawing from towns near Kyiv.

UN Secretary-General Antonio Guterres has called for an independent investigation into the mass killings, and for "effective accountability" to be established.

Ukraine has called on international organizations to send experts to the country as soon as possible to collect evidence of war crimes committed by Russian troops.

On Saturday, Mikhail Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, said the bodies of people with their hands tied and shot dead by Russian troops "lie in the streets" in Bucha.

The Russian war against Ukraine, which started on Feb. 24, has been met with international outrage, with the European Union, US, and UK among others implementing sweeping financial sanctions on Moscow.

At least 1,430 civilians have been killed in Ukraine and 2,097 injured, according to UN estimates, with the true figure feared to be far higher.

More than 4.1 million Ukrainians have also fled to other countries, with millions more internally displaced, according to the UN refugee agency.