GOMA, Congo—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.

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.