United Nations Secretary-General António Guterres has called for an independent investigation into alleged killings in Bucha - the town near Kyiv where bodies were found discarded on a road.
Writing on Twitter on Sunday, he said: "I am deeply shocked by the images of civilians killed in Bucha, Ukraine.
"It is essential that an independent investigation leads to effective accountability."
A statement released by the United Nations Human Rights Office also raised the possibility that Russia had committed war crimes in Ukraine.
"What is known to date clearly raises serious and disturbing questions about possible war crimes and grave violations of international humanitarian law," it read.
It said it was important that all bodies be exhumed and identified "so that relatives can be informed, and that the exact cause of death be established to help ensure accountability and justice".
BBC
Russia's unmanned Luna-25 spacecraft has crashed into the Moon after spinning out of control, officials say.
Donald Trump has confirmed that he will not take part in Republican presidential debates with his rivals in the race for the White House.