Witnesses in the village of Kiwanja saw at least a dozen bodies of adult males, some of whom were burned, apparently hit by rockets or grenades, Reuters reported.
Lt. Col. Jean-Paul Dietrich, spokesman for the U.N. peacekeeping mission in Democratic Republic of Congo, told the news agency that U.N. troops and human rights experts were investigating reports that fighters loyal to rebel Gen. Laurent Nkunda had killed civilians after recent battles with the pro-government Mai-Mai militia.
"Even if they were (Mai-Mai) fighters and surrendered, and were then killed, that would be a criminal act," he said.
Dietrich also said Nkunda's rebels had occupied other villages about 50 miles north of the North Kivu provincial capital Goma. Nkunda advanced to the outskirts of Goma before declaring a cease-fire on Oct. 29, according to Reuters.
Nkunda, meanwhile, told the Associated Press that army forces backed by pro-government militias had attacked the rebel positions in Nyanzale.
"We were attacked three times this morning," he told the AP. "My soldiers have a right to defend themselves."
Fighting has gone on in Congo for years, but it intensified in August and has since driven about 250,000 people from their homes.
Nkunda is seeking direct negotiations with President Joseph Kabila's government, which says it will only meet with all militia groups in the region, not just his.
Dozens of militia groups operate in the remote region of eastern Congo, a lawless area that the government and 17,000 peacekeepers have been trying to control for years.
Among the groups are the pro-government Mai-Mai, and ethnic Hutu insurgents from Rwanda who fled to Congo after helping carry out Rwanda's 1994 genocide, in which about 500,000 Tutsis were killed.
Nkunda, who left Congo's army in 2004, says the Congolese government has not done enough to protect ethnic Tutsis from the Rwandan Hutu militia. Critics say Nkunda has exaggerated the threat to Tutsis and is a puppet to neighboring Rwanda.
A U.N.-backed African Union summit is planned for Friday in Nairobi, Kenya. Kabila, Rwandan President Paul Kagame and U.N. Secretary-General Ban Ki-moon are expected to attend, according to the AP.