KISANGANI, Congo (AP) _ Frustrated by the continuing, random shelling of Congo's third-largest city, the senior U.N. military observer today accused both Rwandan and Ugandan commanders of indiscriminately killing civilians trapped in the ferocious fighting.

At least 50 civilians, 19 of them children, had been killed in the shelling by late Wednesday, Red Cross and other aid officials said. Hospitals have no water, power or medicine to handle the injured. Bodies have been piled in the morgue because it has been too dangerous to bury anyone.

Shells rained down today on this Congo River port for the fourth straight day. On Wednesday, shelling set the Roman Catholic cathedral on the river's northern bank ablaze and shut off water and electricity to some 200,000 trapped Congolese civilians.

Ugandan 107mm shells flew toward Rwandan positions on the south bank of the river. Most of the shells landed in the river, falling short of their target _ Rwandan artillery kept firing at Ugandan forces trying to advance into Kisangani across the Tshopo River bridge, two miles north of the deserted city center.

The fighting defies a U.N.-brokered cease-fire that was meant to demilitarize Kisangani, the main rear base used by Rwanda and Uganda to support rival rebel factions fighting to overthrow Congolese President Laurent Kabila.

``The commanders on the ground are the ones responsible for blocking the cease-fire,'' said Lt. Col. Danilo Paiva, head of the 21-man U.N. military observer mission in Kisangani. ``They are committing genocide against the civilians. They must be held responsible for their actions.''

Paiva said two of the four U.N. observation posts _ houses used by the unarmed monitors to record the rounds fired by each side _ have been hit by mortar rounds, one Ugandan and one Rwandan. There were no casualties.

``There is no valid argument that there need be so many weapons fired,'' Paiva, a Uruguayan, told reporters while taking cover from the shelling. ``Soldiers on both sides are safe in trenches. Civilians die.''

Paiva said he was in contact with the U.S. embassies in Kampala, Uganda, and Kigali, Rwanda, attempting to put diplomatic pressure on both countries to comply with a cease-fire that has already been signed.

James Wakapabulo, a senior Ugandan official, said a 24-hour cease-fire could go into effect if some of the 21 unarmed U.N. military observers stationed in Kisangani would take up position behind the Ugandan lines outside the city.

Col. Karenzi Karake, the senior Rwandan commander in Kisangani, said he had received instructions to stop firing by 4 p.m. Earlier he said the U.S. ambassador to the United Nations, Richard Holbrooke, had been involved in the negotiations.

``We cannot shoot, even in self-defense,'' he said, adding that Rwandan forces were to concentrate at Simi Simi airport while the Ugandans were to assemble at Kapalata barracks outside the city.

Paiva said the commanders had been personally issuing the firing orders for large weaponry, such as a 107mm Katyusha rocket launcher and the BM-21 multiple-rocket launcher, Paiva said.

``The Rwandans placed a battery in the middle of the city. Whenever Ugandans counter the battery, they hit civilians,'' Paiva said. ``They are not fighting in the field. They are hiding in the city.''

Gaspar Mande, 44, has been unable to get home from his job at the Palm Beach Hotel for four days.

``Everybody is talking about Rwanda and Uganda. What about the Congolese? It is as if we don't exist,'' he said, after another sleepless night under heavy bombardment.

Rwanda and Uganda entered Congo 22 months ago as nominal allies to support a single rebel movement. The rebel group split into two factions, and Rwanda and Uganda gradually threw their support behind opposing sides. Last August, they fought a four-day battle in Kisangani in which dozens of civilians were killed. Last month they fought again for several days.

Under a peace accord signed last July in Lusaka, Zambia, all five foreign armies operating in Congo agreed to withdraw under the supervision of a 5,537-strong U.N. force. Troops from Zimbabwe, Angola and Namibia are backing Kabila.

So far, no more than 100 unarmed U.N. military observers and logistics experts have been deployed in Africa's third-largest country.