United Nations staff have been told that nearly 1,000 people were massacred in the Ituri region of north-eastern Democratic Republic of Congo last Thursday.
UN investigators taken to the site at Drodro - about 80 kilometres (50 miles) from the regional centre Bunia - saw mass graves with traces of fresh blood still visible.
According to lists compiled by leaders, 966 people died in three hours of blood-letting.
Forty-nine survivors of the attack in Drodro hospital bore machete and bullet wounds.
The violence came as the warring factions agreed to take part in the Ituri Pacification Commission which is a spin-off of peace accords recently signed in the DRC to end more than four years of civil war.
Ethnic clashes between the Hema and Lendu tribal militias in Ituri have killed thousands of people since the conflict started.
Local groups said the clashes that started out as a simple land dispute between pastoralists and farmers have killed more than 50,000 people and displaced a further 500,000.
Residents of Drodro, a mainly Hema town, said Thursday's attackers spoke a Lendu language.
Hema/Lendu clashes have led to thousands of deaths
According to AFP news agency, the head of the rebel Union of Congolese Patriots (UPC) Thomas Lubanga says the Ugandan army was involved in the killings.
The UPC has been fighting Ugandan troops in Ituri.
But a spokesman for the Ugandan troops in Ituri told the BBC his troops had been far from the site at the time of the incident.
"What benefit would it be to us to massacre civilians," he asked.
"For what reason would the Ugandan army want to infiltrate militias?" he said.
Ugandan troops would stay in the area
"for the protection of the people", he added, even though there is a 24 April deadline for their withdrawal.
DRC authorities urged the UN to find the killers.
"[United Nations representatives] which have already gathered some information on the massacre must quickly pursue its investigation so the perpetrators don't remain unpunished," DRC's Human Rights Minister Ntumba Luaba told AFP.