In a country where one child in eight dies before age 5, the United Nations asked Congo's warring parties for one small favor: a brief cease-fire so children could be immunized against polio. The crippling disease, an anachronism in most of the world, lurks ominously in Congo, where 1,000 cases struck one region just four years ago.

Most of the belligerents in Congo's year-old civil war respected the "Days of Tranquillity," as U.N. Secretary General Kofi Annan termed the cease-fire. But in the remote interior city of Kisangani, the armies of Rwanda and Uganda--which are backing rival Congolese rebel groups--started a firefight in the middle of an immunization campaign.

"It was particularly bad timing on their part," said Martin Mogwanja, the Congo representative for UNICEF, the U.N. children's agency that set out with the World Health Organization to immunize 10 million Congolese children over the weekend. "It affects children; it affects the services we can provide to children. But even worse, the judgment of the international community of the motives and the actions of the armed groups will be relatively serious."

Annan expressed dismay at the fighting after he had received assurances from all parties that they would lay down their weapons on Aug. 8 to permit the polio immunization campaign to proceed Aug. 13-20, the Associated Press reported.

Scores of Congolese mothers and children spent Sunday night trapped in the vaccination centers while artillery, mortar and rifle fire flew in Kisangani's downtown. The fighting knocked out electricity, putting at risk 3 million doses of polio, measles and diphtheria vaccine--which must be kept refrigerated--worth $1.3 million.

Just outside town, a pitched battle for control of the city's larger airport left the United Nations with no way to reach more remote sections of a country roughly the size of the United States east of the Mississippi River.

The consequences for the immunization campaign--months in the planning--were plain. In the approximately 280 of 310 districts that health workers could reach, officials said "close to 100 percent" of children under age 5 were immunized.

But in the Kisangani area, where health workers expected to drip two drops of vaccine onto the tongues of about 80,000 children, 70 percent received doses before fighting sent civilians scurrying for cover. As of two days ago, at least 50 unarmed people were reportedly killed in the shelling and light-arms cross-fire, according to news services.

Late today, a lull in the fighting was declared an official cease-fire. Rwandan Vice President Paul Kagame and Ugandan President Yoweri Museveni emerged from overnight meetings at a safari lodge in southwestern Uganda with a communique that ordered a halt to an episode that has thrown a cloud over both nations.

"The temporary madness in Kisangani is at an end," said John Nagenda, a Museveni adviser.

The two countries, which are allied with each other as well with the United States, moved troops into Congo last August, in concert with a military rebellion against the Congolese president, Laurent Kabila, they had helped install two years earlier. Rwandan and Ugandan forces now control about half of Congo.