The trouble began here on Tuesday, at the height of a hot day, when five ethnic Albanians allegedly strolled up to a Serb at work in a field south of town and shot him in the chest. The 24-year-old farmer died soon after.

Over the next four days, U.S. troops stationed here as part of NATO's peacekeeping force in Kosovo had to contend with grenade attacks that wounded more than 40 people and too many house fires to count, as ethnic Albanians lashed out at Serbs they believed responsible for atrocities against them and Serbs countered with reprisals of their own. And the situation is likely to get worse, say Vitina's Serbian and ethnic Albanian co-mayors -- who have never been on speaking terms and won't sit in the same room with each other.

The attacks marked a sudden flare-up of violence in what had been a relatively quiet corner of Kosovo, part of a series of attacks reported throughout this Serbian province last week by NATO and U.N. officials. The clashes underscore how difficult it is just to maintain an uneasy peace in the aftermath of a bitter ethnic war, while at the same time trying to reconstruct some form of local government.

"We were making a lot of progress, trying to get people focused on getting back to normal life rather than on hatred between Albanians and Serbs," said U.S. Army Capt. Matt McFarlane, who commands 120 troops stationed in Vitina. "This has forced us to take two steps back."

Vitina, about 25 miles southeast of the provincial capital, Pristina, is a dusty, ragged town that had 9,000 residents before Serb-led Yugoslav forces began killing or expelling Kosovo's ethnic Albanian majority earlier this year. With the return of Kosovo Albanian refugees after a 78-day NATO air offensive against Yugoslavia, Vitina's Serbian population has dropped from perhaps 30 percent to 15 percent of the whole, and more are leaving every day.

In addition to the slain farmer this week, a Serbian woman was wounded in a grenade attack on a cafe, a small explosive was thrown at the Serbian co-mayor's house, and numerous Serbian homes in and around Vitina have been burned in arson fires. U.S. peacekeepers also arrested 13 members of the separatist Kosovo Liberation Army -- an ethnic Albanian guerrilla group that agreed to disarm after the war -- when they discovered a cache of gasoline bombs and guns here.

Serbs in Vitina, like those in many other Kosovo towns, complain that NATO -- until last month locked in battle with the Serb-controlled Belgrade government -- favors ethnic Albanians in disputes and does little to protect the Serbian minority from attacks.

"To be honest, [NATO forces] are the ones that have created this situation," said Dragan Kojic, a Serbian Orthodox priest who said he was on his way to the farmer's funeral. "We have concluded that we will wait a couple days, and if it's still not safe, we will all flee from here."

McFarlane and other U.S. officials say they take pains to treat both sides fairly in all disputes and that ethnic Albanians are not responsible for all the recent violence. On Thursday, officials said, a Serb was seen lobbing a grenade into an ethnic Albanian crowd on market day. Thirty-eight ethnic Albanians and two Serbs were injured.

Agron Hoxha, 27, a former KLA field commander who was installed by the separatist group as the town's ethnic Albanian co-mayor, asserted that most of Vitina's Serbs had participated in killings, arson and other crimes during the war. He also declared that the Serbian co-mayor, Vesko Piric, should be arrested for encouraging violence against ethnic Albanians.

"They are aware of their responsibility, and that is why they are afraid," Hoxha said. "It is a hard thing to forget the things they have done. . . . I would prefer that the ones that have been responsible for atrocities" leave the province.

Piric denies wrongdoing, saying that local Serbs have been wrongly accused of many things. "The Albanian terrorists are responsible for this," he said using a common Serbian characterization of the KLA.

Given such tensions, McFarlane and his troops say progress in rebuilding Vitina's government and infrastructure has been curtailed. About 20 people from each ethnic group have been assigned temporarily to city hall administration, although the Serbs rarely show up and a seven-member committee had to be formed just to supervise the opening of a state-run gas station.

The eruption of violence this week, McFarlane said, only makes the peacekeeping job harder. "I was very optimistic until the incident on Thursday," he said. "It just sent it all home: Who would throw a grenade on market day with all those kids around? It's hard for me to imagine."