One or more gunmen attacked a group of U.S. Marines at their headquarters in the southeastern Kosovo city of Gnjilane this evening, and the Marines shot and killed a presumed assailant on a sidewalk outside, according to U.S. military officials.

The incident was the second this week in which Marines in Kosovo have come under attack. In an assault on a Marine checkpoint on Monday, one Serb was killed and another wounded.

No Marines were injured in today's attack, which occurred outside a former government office building where several dozen U.S. military officers are stationed.

But the shooting shortly before 6 p.m., came on a day when the killings that have characterized the past few days of sporadic violence in Kosovo seemed to abate. No politically related murders were reported; 14 people were reported killed on Thursday.

Looting in the Serbian province's five largest cities continued, however, and Italian soldiers arrested several looters in the city of Pec.

On the outskirts of Pristina, the capital of Kosovo, British troops came under sniper fire. They shot back but failed to capture or wound their assailant. In the southern city of Prizren, three members of the ethnic Albanian rebel force, the Kosovo Liberation Army, were arrested by German troops for carrying pistols and hand grenades.

In an interview in Belgium, NATO commander Gen. Wesley K. Clark said Kosovo urgently needs an international police force to work with the 21,000 allied troops to restore public order. "Long experience in peace operations has proved that good soldiers, no matter how well equipped, trained, organized and led cannot fully perform police duties among local populations," he told the Reuters news agency.

There were signs, however, that some aspects of life were returning to normal. In Pristina, for example, a handful of cafes have reopened despite shortages of coffee, milk and sugar that have driven up prices. Two vegetable markets have reopened, and several damaged bars or restaurants have made enough repairs to begin serving a growing clientele.

Ethnic Albanian refugees continued to stream into Kosovo from Macedonia and Albania. The influx on Thursday was estimated at 48,000, a rate that U.N. refugee agency spokeswoman Paula Ghedini described as "one of the largest spontaneous returns that we have ever seen in the last 25 years of any operation."

The number of ethnic Albanians who have returned since the June 10 signing of a peace accord between NATO and Yugoslav military leaders is estimated at more than 300,000, leaving more than 500,000 refugees outside Yugoslavia. The pace has overwhelmed the United Nations' plans to help organize the refugees' return and to ensure that they have adequate food and shelter.

However, its refugee agency has tentatively said it will begin transporting refugees back from Macedonia by train on Monday -- effectively reversing the organized expulsions of ethnic Albanians that Yugoslav troops orchestrated with trains in April and May.

As refugees have returned, tens of thousands of Serbs have fled Kosovo in fear of reprisals for the looting, executions and house burnings performed by government troops during NATO's 78-day air campaign. Several hundred Serbs who had holed up in a monastery in Pec left the city under NATO escort, while about 600 Serbs and Gypsies in the town of Kosovo Polje outside Pristina held a demonstration to protest NATO's occupation and to debate whether they should flee.

In Pristina, a booby trap left behind by Yugoslav troops killed an ethnic Albanian who tried to force his way into a locked apartment.

Brig. Gen. John Craddock, who commands American forces in the eastern portion of Kosovo, said this week that similar explosives have been discovered by his troops.

"We've found buried antipersonnel mines with trip wires. We've found mines that appear to have been thrown out as forces withdrew toward Serbia along roadsides that are not marked on maps provided by the Serb forces," he said. "We're also beginning now to find booby-trapped areas. Some of the towns where [Yugoslav] forces were garrisoned, we're finding that they left ordnance, they left weapons, they left ammunition, and much of that has been booby-trapped."

CAPTION: At a Pristina apartment building, a resident watches British paratroops arrest three armed men who had been harassing the building's Serbs.