Kin-Kiey Mulumba, a spokesman for the Rwandan-backed Congolese rebels, said government troops had attacked rebel positions at Ikela and Opala, two strategic towns 430 miles east of the capital, Kinshasa.
Mulumba had no details or casualty figures. But Rwandan army sources, speaking on condition of anonymity, said fighting broke out Friday when government troops or their Zimbabwean allies sneaked behind positions held by Rwandan troops outside Ikela, attempting to surround them.
"This is a clear violation of a cease-fire, and we are forced to respond," Mulumba said.
Ntuaremba Onfre, the Congolese official responsible for relations with the 100 U.N. liaison officers and military observers now in Congo, said in Kinshasa that cease-fire violation claims were "false information designed to inflame the situation." There has been no shooting, he said.
Meanwhile, Congolese Foreign Minister Abdoulaye Yerodia rejected a proposal by former Botswana president Ketumile Masire to hold a national dialogue in July to chart Congo's future political course.
Masire, Yerodia said, should "keep his ideas in his pocket" and accused him of diplomatic meddling.
The national dialogue was included in a peace accord signed last year. Yerodia said the government was disappointed the conference had not taken place already.
On Friday, presidents from Uganda, Namibia, Zimbabwe, Mozambique and South Africa meeting in Victoria Falls, Zimbabwe, reaffirmed their commitment to a cease-fire and a withdrawal of foreign troops from Congo, which is supposed to be supervised by a 5,500-strong U.N. observer mission.
Rwanda and Uganda back the rebels. Zimbabwe, Namibia and Angola back Kabila.
A new cease-fire took effect in Congo on April 14. Despite initial reports of fighting, U.N. observers and a joint military commission made up of warring sides have said it was generally holding well.
The United Nations is yet to obtain the green light from the government in Kinshasa for the deployment of U.N. personnel.
Richard Holbrooke, U.S. ambassador to the United Nations, is expected in Congo early next month to seek additional security guarantees for U.N. forces, which are to deploy within a designated 18-mile buffer separating the warring sides.