Alan Doss, who heads the United Nations mission in the Congo, said the slayings in the African nation's eastern region were "serious violations of human rights and international humanitarian law" that "constitute war crimes." Doss leveled the accusations while in Goma before returning to Nairobi, where African leaders were meeting on the Congo crisis.
Forces loyal to renegade-general Laurent Nkunda and pro-government militia members have clashed since Tuesday in Congo's North Kivu region.
Doss implored all armed groups to withdraw from the area around Rutshuru to allow peacekeepers to protect civilians in the vicinity and enable the safe return of thousands who have fled.
African leaders who met Friday in Nairobi said they want a cease-fire in the Congo. They said peacekeeping troops there should be granted greater powers.
The conflict could be widening, drawing in Angola to fight on the side of the Congolese government against the Tutsi rebels under Nkunda. Some U.N. officials say the rebels are being supported by the government of Rwanda, the BBC said.
Two witnesses have told the British broadcaster they have seen Angolan troops in the area.