It is not clear who carried out the attacks, which occurred in Ituri province, the scene of some of the most vicious battles in Congo's 4 1/2-year-old civil war. Rival tribal fighters, rebel factions and Ugandan troops all have been involved in the fighting in the mineral-rich province.
Witnesses told the U.N. investigators that the attackers included women and children while others were men in military uniforms, said Manodje Mounoubai, a spokesman for the U.N. mission in Congo.
"This is the worst single atrocity since the start of the civil war." Mounoubai told The Associated Press by telephone from Kinshasa, Congo's capital.
The killing spree occurred over a period of just a few hours Thursday in the Roman Catholic parish of Drodro and 14 surrounding villages in Ituri.
"The attack started with a whistle blow and lasted between five and eight hours," Mounoubai told The Associated Press by telephone
U.N. military observers visited the area Saturday and spoke to witnesses, survivors and local leaders who led them to 20 mass graves, Mounoubai said.
He said many of the victims appeared to have been summarily killed.
Another spokesman for the U.N. mission, Hamadoun Toure, said the mass graves had "fresh blood on them." Investigators said some of the survivors were seriously wounded, mostly by machetes but also by bullets.
On Saturday a Congolese rebel leader, Thomas Lubanga, accused Ugandan troops and allied Congolese tribal fighters of carrying out the slaughter.
Lubanga, who is head of the rebel Union of Congolese Patriots, or UPC, said Ugandan troops and Lendu tribal fighters used mortars, small arms and machetes to attack three towns in Ituri, killing 942 people.
Ugandan military spokesman Capt. Felix Kulayigye denied that any Ugandan troops were involved in the massacre. Kulayigye, who is based in Ituri, said Saturday that some 400 people were killed in tribal fighting.
An aid worker and a tribal leader in Bunia, however, said Ugandan forces were in the area when civilians were killed. They could not say whether the troops took part.
Witnesses told the investigators that some of the assailants were speaking Kilendu, the Lendus' tribal language, while others spoke Kiswahili, the lingua franca in eastern Congo, Mounoubai said. Most Ugandan soldiers also speak Kiswahili.
The UPC draws its support from the Hema tribe that has traditionally fought with the Lendu for control of land and other resources in the region.
The UPC and Ugandan troops have been fighting since Ugandan forces drove the rebels from Bunia, the main town in the province, four weeks ago.
Thursday's attack is the latest in a long line of atrocities that have beset Ituri.
On Jan. 15, U.N. investigators confirmed that rebels of the Congolese Liberation Movement and the allied Congolese Rally for Democracy-National had carried out cannibalism, rape, torture and killing in the province late last year.
And on March 1, Lubanga accused another rebel group, the Congolese Rally for Democracy-Liberation Movement, of killing at least 400 civilians when it pushed UPC forces out of Bogoro, 25 miles southwest of Bunia on Feb. 24-25.
The war broke out in August 1998 when Rwanda and Uganda sent troops to back rebels seeking to oust then-President Laurent Kabila. They accused him of backing insurgents threatening regional security. They had earlier supported Kabila's successful bid to oust dictator Mobutu Sese Seko.
Zimbabwe, Angola and Namibia sent troops to back Kabila, splitting the country into rebel- and government-held areas.
Most foreign troops withdrew after a series of peace deals took hold, but fighting among rival rebel factions, tribal fighters and Ugandan troops has continued in eastern and northeastern Congo.
Uganda had backed the UPC, but relations have soured in recent months after the rebels demanded the withdrawal of Ugandan troops from the region. The UPC is now backed by Rwanda.
Since the war began, Congolese rebels have split into more than a dozen factions. Uganda, which has more than 2,500 troops in Ituri, and Rwanda back rival groups.