Thousands more ethnic Albanian refugees, undeterred by the threat of land mines and severe food shortages, poured back into war-ravaged Kosovo today in jubilant convoys that have alarmed relief officials who had hoped to organize a more orderly return.

At least 15,000 refugees crossed into Kosovo at the Albanian border here today, and another 5,000 entered from neighboring Macedonia. The reverse migration came amid the discovery of more sites of atrocities apparently committed during the Serb-led terror campaign in Kosovo. British officials today estimated the campaign took the lives of 10,000 civilians.

In London, the British Foreign Office said Belgrade government forces carried out more than 100 massacres in Kosovo during NATO's 78-day bombing campaign against Yugoslavia, which ended last week. "Tragically, our estimates of the numbers of innocent men, women and children killed will almost certainly have to be revised upwards," Foreign Office Minister Geoff Hoon said.

In Helsinki, meanwhile, U.S. and Russian negotiators struggled to reach agreement on terms for Russian troops to join the international peacekeeping force in Kosovo, where a jurisdictional impasse between NATO forces and a small contingent of Russian soldiers has complicated the stabilization effort. [Story, Page A33.]

As ethnic Albanians surged back into Kosovo from the south and west, thousands of Serbian civilians fled north into Serbia proper. The U.N. World Food Program estimated that as many as 50,000 Serbs -- more than a quarter of the Serbian population of the province -- have left Kosovo since Yugoslav troops and Serbian police began withdrawing.

The exodus of Serbian civilians appeared to be accelerating today, with northbound roads jammed for miles with convoys of cars and tractor carts loaded with television sets, refrigerators and other household goods.

The departure of so many Serbs -- including many government employees -- has left Pristina, Kosovo's capital, without water for three days because workers abandoned the city's water supply system. Mounds of garbage line streets because no one is left to collect it.

NATO officials in Pristina said that as of today, 35,000 government troops and policemen had left Kosovo and that they expect all military and paramilitary forces deployed in the province to meet the midnight Friday deadline for withdrawal from the second of three zones agreed on by the Belgrade government. A NATO spokesman said about 11,000 troops and police remain in Kosovo; all are required to leave by midnight Sunday.

Refugee and relief officials had pleaded for time to arrange a planned, orderly repatriation of the roughly 860,000 ethnic Albanians who were pushed from Kosovo during Belgrade's violent 2 1/2-month-long expulsion campaign. But with government forces departing and NATO troops pushing in, thousands of refugees apparently saw no reason to continue living in crowded camps just a few miles from home across virtually open borders.

"The celebration in Prizren [Monday] struck such a huge, emotional chord with everybody," said Rupert Colville, a spokesman for the U.N. High Commissioner for Refugees, referring to the liberation frenzy that gripped Kosovo's third-largest city, 12 miles from this border outpost. "They saw it on the TV, and they heard it on the radio, and the pull is completely overriding the fear of mines and [unexploded] bombs."

But many of the Kosovo Albanians are returning to ruined villages, where relief supplies cannot be delivered because of continuing NATO security concerns as Yugoslav troops carry out the final stages of a reluctant retreat. Officials with the U.N. refugee agency said they have now determined that some regions of Kosovo have been so utterly devastated that the reconstruction kits they had planned to give returning refugees to help them rebuild their homes are hopelessly inadequate.

U.N. shelter experts made observation flights over parts of Kosovo this morning and "were shocked by what they saw," said Ron Redmond, a spokesman for the U.N. refugee agency. "We're going to have to give them tents."

U.N. officials said they also were concerned about the hundreds of thousands of ethnic Albanians who hid in Kosovo during the government expulsion campaign and are now returning to their towns from forests and mountains hungry and exhausted. Making food and other relief supplies available to them has been complicated by the discovery of booby traps apparently left by departing government troops at U.N. warehouses and offices in Pristina and Prizren, and by massive military and refugee traffic jams on both sides of the Kosovo-Albania frontier.

"The situation is really dire inside Kosovo," said Paula Ghedini, a U.N. spokeswoman. "We did a [food] distribution yesterday in Glogovac [about 15 miles west of Pristina], and within 45 minutes it went from being a ghost town to having thousands of people coming out of the woodwork, crawling down from the wooded areas -- almost all women and children -- and they said they were down to just cornmeal at night. Most were absolutely desperate."

Just outside the Albanian town of Kukes, the World Food Program was giving refugees subsistence packages containing 110-pound bags of flour, oil, beans, rice and canned meat and fish. Program spokeswoman Anna Di Lellio said her group is encouraging refugees to share the food. "This is the best way to get food to the villages," she said.

Despite the deep concern and apprehension of refugee officials about the haphazard repatriation, the mood here in Morina was upbeat, with wagonloads of Kosovo Albanians smiling and waving in obvious relief as they headed back to their homeland. "We will live even in plastic sheets just to be home," said Asime Mugaj, 28, who was riding with 21 friends and relatives in a trailer attached to a tractor that was being towed by a dump truck with another 48 people in its bed.

"Kosovo! Why not!" shouted an ecstatic Bahit Sadik, 70. "I will go there and eat sweets and milk in celebration!"

But it was not a happy day for all the refugees. About 10 a.m., Azem Saramati, 50, and his wife Shyqerie, 46, shuffled slowly against the traffic, heading back from Kosovo toward their erstwhile tent home in Kukes. They had gone home Wednesday to check conditions in their town and found their three-story house burned to the ground.

"We're going back to the camp, and we'll stay there until they tell us we have to go," said Azem, whose brother -- a guerrilla in the ethnic Albanian Kosovo Liberation Army -- died in battle with Yugoslav forces 10 days ago. "Maybe we can live in the basement. The floor looked okay."

While hundreds of Kosovo-bound cars, tractors, trucks and buses lined the road as far as the eye could see, hundreds of men walked in the opposite direction -- going to Albania to find their families and take them home. Some, like the Saramatis, had gone back Wednesday on reconnaissance missions. Most were members of the KLA, or men who had hidden in Kosovo during the war and sent their families to Albania for safety.

"My village was destroyed and burned, and the Serbs looted everything," said Adem Gashi, 28, a KLA guerrilla who was headed for a refugee camp outside Kukes. Only one room of his house had survived -- not enough for the 12 people in his family. "We will live in a tent in our garden. I don't know what we'll do for food."

All of the men were walking because the Albanian government had closed in-bound traffic lanes from 3 a.m. until 6 p.m. to allow for the heavy flow of outbound refugees. That created traffic gridlock in Kosovo, where cars headed to Albania lined up 18 hours ahead of time, waiting to cross the border. With German tanks and other NATO armored vehicles trying to pass as hundreds of broken-down cars and tractors tried to move in the other direction, chaos reigned. And it was not much better across the border, where motorists said it had taken them five hours to reach Morina from Kukes -- a trip that usually takes about 35 minutes.

Relief officials had hoped to have emergency road repair and tire centers set up along the route, but that program was another casualty of the hasty return. Instead, the Irish aid group GOAL reportedly planned to give tow ropes to every 20th vehicle.

But the principal concern among refugee officials remained the danger of land mines in Kosovo. A large billboard at the border, emblazoned with red skulls and crossbones, warned returning refugees of the danger, while U.N. workers handed out leaflets to each car. The Red Cross said it was printing 200,000 pamphlets for refugees as well.

"Many people are going back to roadside villages that we know for a fact are mined, and we're afraid of booby traps that are attractive for someone to pick up, especially children," said U.N. spokeswoman Ghedini. "People right now have a heightened sense of confidence, and only when they see that mines are a real threat are they going to be wary."