Tribe claims 2,400 have been killed in attacks by Ugandan-backed rebels

United Nations observers are investigating claims that at least 2,400 people in north-eastern Congo have been massacred in attacks by a rival tribe and its rebel allies who are backed by Uganda.

The inquiry into alleged attacks on the Hema tribe by Lendus was launched yesterday despite threats against UN workers and non-governmental organisations in the area by members of both communities, said Manodje Mounoubai, a spokesman for the UN mission in the Democratic Republic of Congo.

The UN has eight unarmed observers in Bunia, where the rebels are based.

"We cannot let these threats prevent us from acting," Mr Mounoubai said. "Unless there is imminent danger, we cannot stop working." He added that the commander of Ugandan troops in the area had confirmed that UN staff were at risk and had offered to beef up security around Bunia, which is about 980 miles from Kinshasa, Congo's capital.

But by yesterday the UN troops had been unable to leave the town to investigate the reported killings, because the Ugandan troops had refused to provide an escort. Mr Mounoubai said the UN would try to negotiate access to remote villages later this week.

A Hema spokesman said that at least 2,400 Hema have been killed since April during a series of raids by the Lendu tribe and its rebel allies, the Ugandan-backed Congolese Rally for Democracy-Liberation Movement.

Reports of massacres in remote areas of Congo are not infrequent, but at times rival groups appear to level or exaggerate claims of atrocities for political ends. The tribal conflicts tend to be distinct from Congo's civil war, though at times the differences become blurred.

The UN force commander Major General Mountaga Diallo said on Monday that many villages had been burnt around Bunia, but he could not provide casualty figures. The UN force's mandate does not allow it to intervene in tribal conflicts.

Lendu tribe members could not be reached for comment, but Sumaili Koloso, secretary general of the Congolese Rally for Democracy-Liberation Movement said the fighting was "a competition for leadership [rather] than an ethnic war". He said fighting was taking place between his group and a militia formed by his former defence minister, Thomas Loubanga, a Hema.

The Hema and Lendu communities have fought periodically for decades over tea and coffee farms and cattle. The use of primitive weapons kept casualty figures low in the past. But following the outbreak of Congo's war in August 1998, the communities have been able to arm themselves with modern weapons.

The civil war broke out when rebels backed by Uganda and Rwanda sought to overthrow the late President Laurent Kabila.