Dozens of armed militiamen have attacked the United Nations headquarters and a military camp at Bunia in north-eastern Democratic Republic of Congo, injuring a military observer.
About 4,000 peacekeepers are in the Bunia area
UN forces sent to the region to try to end tribal conflict had returned fire, a spokesman in Bunia said.
Witnesses in Bunia said UN troops and the militiamen were involved in a gun battle that lasted for two hours during the night in the town centre and continued until dawn in several areas.
In another development, the UN has protested to the DR Congo Government after being prevented from inspecting a site where a plane the UN suspects of flying in weapons had crashed.
The situation in Bunia was described as being tense but calm on Thursday after the overnight clashes.
"We can't yet say who attacked and we don't yet know how many victims there were on the side of the assailants," said UN spokesman Leocadio Salmeron.
Leaders of the various militia groups in Bunia have denied any involvement in the attacks, which also targeted the house of Emmanuel Leku, the chief of the UN sponsored Congolese administration in Ituri province.
However some Bunia residents told the BBC that one of the main rebel groups there, the UPC, was to blame.
The gunfire erupted after the UN had intervened to disband a UPC meeting.
More than 4,000 UN troops have been deployed in Bunia since September in the latest effort to end fighting between Hema and Lendu ethnic groups which has claimed an estimated 50,000 lives in the past two years.
The UN mission, known as Monuc, sent a team of military observers to Kamina, in South Kivu Province - which borders Rwanda - after a plane crashed on take-off there at the weekend.
The UN wanted to check reports that the plane might have been used to ferry supplies to armed fighters in the east of the country.
In a statement, Monuc said observers were barred from approaching the site which was guarded by soldiers and a number of civilians.
They were also unable to locate
the Russian crew or a Congolese officer known to be aboard the aircraft.
Monuc suspects the plane of bringing weapons into the volatile east of the country, but authorities in the area insist the plane crash involved a civil aircraft with no secret cargo.
UN forces have been blocked several times recently by armed fighters as they seek to enforce
peace after five years of war in a nation the size of Western Europe.
Monuc is monitoring a ceasefire that took effect in April to end a regional war that broke out in 1998 when rebels backed by Rwanda and Uganda sought to overthrow the Kinshasa government.
However, the east has remained volatile, shaken by sporadic fighting and ethnic massacres.