Some 40 of the 65 people killed in north-eastern Democratic Republic of the Congo were children, a United Nations spokeswoman has said.
The UN fears that more bodies will be found (Pic: UN)
UN peacekeepers reached the massacre site of Kachele, 100 kilometres (60 miles) north-east of Bunia, capital of the troubled Ituri province, on Tuesday.
They found bodies, killed by bullets or machetes, in five mass graves and lying in the nearby bush, Isabelle Abric said.
Ethnically-based militias have killed more than 50,000 in the past four years in the Ituri region, but this is the first reported massacre since the UN peacekeepers took over from a European Union force a month ago.
Hamadoun Toure, spokesman of the UN mission in DR Congo (Monuc), says that more than 65 people may have been killed in this week's attack.
"Information is coming in bit by bit about the massacres but we can confirm that Katshelli was only one of four or five hamlets in close proximity hit by the attack," he said.
Both the EU and UN forces have been based in Bunia but the UN representative in DR Congo William Swing, said that UN troops would start deploying outside the capital by next week at the latest.
The BBC's Arnaud Zajtman in the capital, Kinshasa, says the UN will first determine which ethnic group the victims came from, which may point to who was responsible.
UN troops are hunting for the killers (Pic: UN)
A leader of the Union of Congolese Patriots (UPC), an ethnic Hema group backed by Rwanda, blamed rival Lendu fighters for the killings.
"It's the Lendus who have attacked," UPC leader Thomas Lubanga told Reuters news agency.
He said that the two main Lendu groups, the FNI and the FRPI, were responsible.
Kachele saw fierce fighting between Lendu and Hema groups in July and August, with many civilians displaced by the unrest.
Almost 3,400 UN peacekeepers are now deployed in Bunia, and a military spokesman said last week that since their arrival in early September, there had not been any large-scale killings in the Ituri region.