Reports that Angolan troops joined Congolese soldiers battling rebels near the city of Goma raised new fears the conflict could spread in the region, but the U.N. chief holding a peace conference in nearby Nairobi denied the reports.

New clashes between the army and rebels erupted outside Goma near Kibati, where about 45,000 refugees from the rebellion in mineral-rich eastern Congo have taken refuge. Thousands fled toward the relative safety of Goma.

The French aid group Doctors Without Borders reported fighting in the towns of Rutshuru and Kiwanja, where the charity tried to send staff who had to turn back. The aid group said Rutshuru hospital was full of displaced civilians.

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Congo asked Angola for support Oct. 29, as rebels led by Tutsi former general Laurent Nkunda advanced toward Goma, capital of North Kivu province near the Rwandan border. Nkunda called a unilateral cease-fire last week when his forces reached the city outskirts, but the truce has crumbled.

A U.N. official and a Uruguayan peacekeeping officer said Friday that an unspecified number of Angolan troops arrived four days ago. The two officials spoke on condition of anonymity because of the subject's sensitivity.

But U.N. Secretary-General Ban Ki-Moon said he knew nothing about Angolan troops in Congo.

"I have no information about the Angolan troops participating in this at this point," Ban said at a news conference called after a peace summit in Nairobi, Kenya.

He then asked U.N. envoy in Congo Alan Doss, who replied "No, Secretary-General ... I have no direct independent confirmation of that report." In New York, U.N. Assistant Secretary-General for Peacekeeping Edmond Mulet suggested some people may have mistaken Congolese government troops who had trained in Angola, and therefore spoke Portuguese, for Angolan troops.

The involvement of Angolans could spread the conflict beyond Congo's borders. Neighboring Rwanda likely would consider Angolan troops a provocation. Rwanda's Tutsi-led government is accused of supporting the Congolese rebels.

Congo's 1998-2002 war drew in more than half a dozen African nations, including Angola and Rwanda, which profited from the vast country's wealth of diamonds and other minerals.

Meanwhile, African leaders criticized the U.N. peacekeeping force in Congo — the world's largest — for failing to protect civilians and end the violence that is convulsing eastern Congo.

Ban, holding the peace summit with Congolese President Joseph Kabila and six other African leaders, warned the "crisis could engulf the broader sub-region."

"We must put the cycle of violence behind us," Ban said, while calling for the Congolese army to be strengthened to respond to the situation.

Kabila spokesman Kudura Kasongo said the U.N. mission in Congo, known as MONUC, has not fulfilled its mandate. "If they have failed, why are we being left alone with that burden?" Kasongo asked.

Delegates at the summit said the region should send peacemaking forces if necessary. The region "would not stand by to witness incessant and destructive acts of violence by armed groups against innocent people," a meeting statement said.

The government has refused direct talks with the rebels, and they were not at the meeting in Nairobi.

But Kabila and Kagame held a rare face-to-face meeting for about five minutes Friday, the spokesman Kasongo said. Kabila's government accuses Kagame's Tutsi-led government of supporting the Congolese rebels.

The conflict is fueled by ethnic hatred left over from the 1994 slaughter of a half-million Tutsis in Rwanda. Nkunda claims he is fighting to protect minority Tutsis from Rwandan Hutu rebels who participated in the genocide and fled to Congo afterward.

The violence in eastern Congo since August has driven 250,000 people from their homes. New York-based Human Rights Watch says at least 100 have died in the last two months.

Also Friday, a U.N. official said several journalists who were in the conflict zone, including four foreigners, are missing or held hostage. Speaking on condition of anonymity because of the sensitivity of the issue, the official said three or four journalists taken hostage by militiamen were handed over early Friday to Congolese army troops and U.N. peacekeepers are trying to reach them, the official said. Three other European reporters have been missing for 72 hours.