The United Nations says new fighting has erupted in the eastern Democratic Republic of Congo, as southern African leaders call for a cease-fire.

A spokesman for the U.N. mission in Congo, Lieutenant Colonel Jean-Paul Dietrich, says rebels led by General Laurent Nkunda battled pro-government militias Sunday around the town of Ngungu in North Kivu province. The clashes lasted about six hours, ending after mediation by U.N. peacekeepers.

In Johannesburg, leaders of the Southern African Development Community called for an immediate cease-fire in the DRC so that aid can get to displaced people.

An international aid organization, Doctors Without Borders (Medecins Sans Frontieres), is reporting an outbreak of cholera in camps for displaced people. The bacterial disease thrives when people are forced to live together in unsanitary conditions. It is simple to cure, but can be fatal if left untreated.

Tens of thousands of people have been driven from their homes or displaced persons camps since Nkunda's force launched an offensive two weeks ago. The rebels control territory north of the provincial capital, Goma.

On Saturday, the chief of the U.N. mission in Congo leveled accusations of war crimes against the groups involved in the fight. U.N. special envoy Alan Doss said Nkunda's men and and pro-government militias have both executed civilians. Officials say at least 26 civilians have been killed in fighting in the region this week.

Fresh clashes in North Kivu broke out in August, after the collapse of a January peace deal between Nkunda's force and the government.

Nkunda says he is fighting to protect the area's Tutsi minority from Rwandan Hutu militias that entered the region after the 1994 Rwandan genocide.

Congo's government accuses the Rwandan government of aiding Nkunda - a charge Rwanda denies.

Some information for this report was provided by AFP and AP.