NATO troops in Kosovo have for the first time arrested a Serbian militiaman suspected of involvement in executions of ethnic Albanians, while a series of looting incidents and apparent revenge killings of Serbs today strained the ability of the 17,000 NATO troops in Kosovo to police the province.

In an atmosphere of rising lawlessness, a Serbian faculty member and two other employees of the University of Pristina were slain this morning, apparently as part of an effort to force the resignation of all Serbian professors there. Looting continued in urban centers, as more and more ethnic Albanian refugees returned to Kosovo to find their homes destroyed and only those shops or houses owned by Serbs left intact.

NATO military police and regular troops are intervening frequently to stop looting and have opened murder investigations in hopes of building confidence among ethnic Albanians and the remaining Kosovo Serbs that law and order will prevail. The British troops' arrest of the Serbian militia member Wednesday evening provided the first indication that some allied units may be prepared to take an activist approach to hunting down and detaining war-crimes suspects still living in Kosovo -- a province of Serbia, Yugoslavia's dominant republic.

The policing tasks have fallen to NATO troops in the security vacuum created by demilitarization of the province under a peace agreement the alliance reached with the Belgrade government June 10 and a subsequent deal banning the ethnic Albanian Kosovo Liberation Army from wearing uniforms or carrying weapons in public, except in designated areas.

A force of 1,500 international policemen is slated to arrive here next week, but diplomats say the group is not large enough to make a substantial dent in the backlog of criminal cases arising from the war over Kosovo and its ensuing chaos. No courts are functioning here, and a U.N. effort to set up a civilian government structure remains in it infancy.

U.S. Army General Wesley K. Clark, the NATO supreme commander, said today in Pristina, Kosovo's capital, that he has asked nations that have promised to send additional troops to take part in the peacekeeping effort to "speed the deployment to the maximum extent possible." After consultations with British Lt. Gen. Mike Jackson, the NATO commander in Kosovo, Clark said that the alliance can stick to its earlier target of deploying a total of 55,000 troops but that more need to get here sooner.

An estimated 60,000 of Kosovo's 200,000 Serbs have fled the province since NATO troops arrived, but some of those who remain are fearful due to the return over the past 11 days of 250,000 ethnic Albanians, or nearly a third of the number who fled or were forced from their homes by Serb-led government security forces. As a result, officials here say, sniping incidents involving hard-line Serbs have been increasing; clashes are also becoming more common between returning refugees and Serbs who moved into their vacant homes and assumed squatters' rights.

In the sector of Kosovo patrolled by British troops, a total of 12 shooting incidents occurred today, the largest number since they arrived on June 12. Twice this evening, British soldiers in the capital chased off packs of looters inside shops along Pristina's main street.

Also in Pristina, a gun battle erupted between members of an ethnic Albanian family and the Serbs they found living in their apartment near the city hospital, leaving one man dead and another seriously wounded. When the rival families encountered each other in the hospital, a second gunfight left a security guard and a nurse wounded. British troops arrested both families and confiscated their weapons.

The names of those slain this morning at the University of Pristina were not released by the British police who sealed the site and opened a formal investigation, but one of the dead was identified as a professor of economics. The university has been an ethnic battleground since 1989, when the Serbian government fired all ethnic Albanian professors amid a general political crackdown. British military officials said that two other Serbian professors have been warned anonymously not to continue working there.

In the northern Kosovo city of Mitrovica, French troops began detaining ethnic Albanians found looting the homes of Gypsies, a minority that was allied with Serbs during the war. In Pec, a city of 60,000 where Yugoslav army troops looted and burned virtually every ethnic Albanian-owned home, the remaining Serbian civilians are under heavy military protection.

"I would like to call on all Kosovo Albanians, and indeed on all the other people of Kosovo, not to allow ethnic hatred or the desire for revenge to cut their hearts," NATO Secretary General Javier Solana said.

Here in the village of Slovinje, British military police arrested militiaman Dragisa Peica on tips from ethnic Albanians who returned in recent days. Three witnesses accused him of involvement in the April 15 executions of 43 village residents and the subsequent removal of their bodies from several mass graves in an apparent attempt to conceal the incident from Western war-crimes investigators.

As in neighboring Bosnia, where NATO peacekeepers were deployed in 1995 after a 2 1/2-year factional conflict, the rules of military engagement call for the arrest of suspected war criminals only when they are encountered in the course of normal activities. But villagers here said they were delighted that British forces acted on their tip and said that additional participants in the executions are also at large in the area.

By taking such actions, NATO forces are moving into fairly uncharted legal waters, however, particularly since none of the suspects here has been indicted by the international war crimes tribunal at The Hague. Since this village lies in the sector patrolled by British troops, Peica is being held under provisions of British military law. American, French, and German military legal standards differ, and so the treatment accorded such suspects would depend on where they are arrested.

In Washington, the U.S. government today announced rewards of up to $5 million for help in arresting and convicting anyone indicted by the tribunal. Yugoslav President Slobodan Milosevic and four of his top aides are among those indicted.

U.S. Brig. Gen. John Craddock, who commands Task Force Falcon in the eastern portion of the province, has noted that "there's no law here that governs." He said his own troops have detained three people, including a Serb suspected of a sniping at two members of the rebel Kosovo Liberation Army, which has fought an 18-month guerrilla campaign for Kosovo's independence.

German troops patrolling southern Kosovo have jailed 20 to 40 people in Prizren, but there is no venue for such suspects to be tried. Moreover, some Western officials say the potential number of war crimes here is so great that even the international tribunal will be an inadequate forum to hear them all.

"There is a lot of discussion about how to create a court system [in Kosovo] that would be independent of Belgrade," said David Scheffer, the U.S. ambassador at large for war-crimes issues, during a visit today to the city of Djakovica.

He said the tribunal would not be able to investigate every alleged war crime in Kosovo and that the answer may be to create a local judicial system capable of sharing the load.

Also in Djakovica, the FBI began its first-ever war crimes field investigation, using two teams of investigators to examine forensic evidence where 26 people were killed.

Correspondent Peter Finn contributed to this report.

CAPTION: Bekim Krasnici, a Kosovo Albanian, watches as a Serbian-owned house in the city of Pec burns -- one of a dozen arson fires there as a wave of lawlessness and ethnic reprisals in the province has forced NATO peacekeepers to intervene to try to keep order. (Photo ran on page A01)

CAPTION: An Italian peacekeeper in Pec uses a blanket to try to put out a fire in a house belonging to Gypsies. Ethnic Albanians, who started the fire, have accused Gypsies of collaborating with Serbs in attacking Albanians in Kosovo.