The United Nations peacekeeping force in the Democratic Republic of Congo says at least 30 people were burnt to death in their huts on Saturday night.
UN peacekeepers are tightening the screw in eastern DR Congo
Monuc sent 50 peacekeepers to Mamba village, in South Kivu province, to verify reports it had been attacked by Rwandan Hutu militias.
They discovered a village burnt to the ground and two mass graves.
Witnesses said 39 villagers, mostly women and children, had been locked in their huts which were then set ablaze.
UN officials say at least 50 others were wounded.
The BBC's Arnaud Zajtman in the Congolese capital Kinshasa says the massacre took place 40 km west of Bukavu in the park of Kahozi Biega, a rebel stronghold where UN peacekeepers have only recently begun patrols.
The UN believes the massacre could be a warning to the local population not to co-operate with the peacekeepers.
Much of the South Kivu region is under the control of the FDLR, which is accused of playing a lead role in the 1994 genocide in Rwanda in which about 800,000 Tutsis and moderate Hutus were killed by Hutu militias.
Speaking from his exile in Brussels, the leader of the Hutu rebels, Ignace Murwanashyaka, denied ordering the attack, and blamed a splinter group.
Hutu rebels fled Rwanda and crossed into Eastern Congo 11 years ago after their alleged involvement in the Rwandan genocide.
An estimated 15,000 Hutu rebels are still active and represent one of the main threats to security in the area.
Several rounds of negotiations and a UN-sponsored voluntary disarmament programme have failed to restore peace to Eastern Congo.
The rebels say that they will return peacefully to Rwanda only when the political situation allows, but recently the UN peacekeepers and the Congolese army have threatened to use force to disarm the militiamen.