US President Joe Biden has called his Russian counterpart Vladimir Putin a war criminal and suggested holding a war crimes trial.
The Kremlin has categorically denied accusations that its forces have killed civilians in Ukraine.
The Ukrainian government has demanded crippling new sanctions from Western powers over what it called the Bucha massacre.
A team from the International Committee of the Red Cross says it was prevented from reaching the besieged Ukrainian city of Mariupol to evacuate civilians.
Ukrainian President Volodymyr Zelenskyy says he will address the UN Security Council on Tuesday.