At least four gunmen opened fire on a U.S. Marine platoon at a checkpoint near the town of Zegra in southeastern Kosovo early this evening, provoking a lengthy gun battle that left one of the gunmen dead and two wounded with no American casualties, according to U.S. military officials.

The Marines immediately called in reinforcements, including several Cobra attack helicopters, as the gunmen barricaded themselves in a nearby building for several hours. One of the gunmen sustained minor injuries, but another was critically wounded and was operated on by Marine surgeons after being evacuated from the scene.

Initial reports differed on whether the Marines captured one or two additional gunmen. U.S. officials said the Marines believed their attackers were Serbs in civilian clothes but could not say so definitively.

The firefight was the first deadly altercation involving U.S. military forces since they arrived in Kosovo on June 14. On Monday night, Serbian civilians fired at a Marine platoon in eastern Kosovo after it was sent to investigate several nighttime house fires but no one was injured. Officials said two suspects from that incident are now in custody.

This evening's gun battle incident occurred amid new reports of scattered Serbian resistance to the presence of NATO peacekeeping troops in Kosovo and to the return of ethnic Albanians who were forcibly expelled by Serb-led security forces -- all of which have now left Kosovo, a province of Serbia, Yugoslavia's dominant republic.

There were also new reports of reprisal attacks on Serbs. In southwestern Kosovo, ethnic Albanians looted and burned the Serbian settlement of Novake today after its residents had fled the province.

Speaking of today's gunfight, U.S. Brig. Gen. John Craddock, who commands the American Task Force Falcon in the eastern portion of Kosovo province, said the NATO troops face a challenge from a "rogue element" that either does not know "or refuses to acknowledge . . . that agreements have been made. And that's for both sides," referring to ethnic Albanians and Serbs.

Some sniping occurred today here in Pristina, the Kosovo capital, shortly before a visit by British Foreign Secretary Robin Cook, German Foreign Minister Joschka Fischer and French Foreign Minister Hubert Vedrine, although no injuries were reported, officials said.

In Kosovska Mitrovica, one of the last cities from which Yugoslav troops withdrew, armed Serbian civilians succeeded for a third day in blocking ethnic Albanians from crossing the Ilber River to reach food shops or return to their homes in a neighborhood where the majority of residents are Serbs.

One reason for the heightened tensions is the increasing flow of returning refugees. Nearly a quarter of the estimated 860,000 ethnic Albanians who fled or were forced into neighboring nations by Yugoslav troops and Serbian police have now reentered Kosovo, according to U.N. officials -- a total of about 206,000.

Relief agency officials say they are bringing in enough food to feed 20,000 to 30,000 new returnees people daily. "So far, we're keeping up with food requirements," said Paula Ghedini, a spokeswoman for the U.N. High Commissioner for Refugees. But she added that the scale and speed of the migration has prompted fresh concern that some returning refugees will be unable to find adequate shelter, particularly in such cities as Pec and Djakovica, where thousands of homes were torched by government security forces.

In Mitrovica, French troops responsible for patrolling the city have said they need not intervene in the dispute over access to the Serbian neighborhood. Today, they escorted one ethnic Albanian past the Serbian civilians after he began crying in the presence of reporters. But French Defense Minister Alain Richard defended the stance taken by the troops after a morning visit to the city, saying that most Serbs and ethnic Albanians there had lived separately before the conflict and that the peacekeeping force was meant to help all Kosovo residents "live in safety."

Vedrine said in Pristina later in the day, however, that French forces would eventually "apply the same rules there" as those applied by other NATO forces in Kosovo. He also noted that the peacekeeping force is "still in the early stages" of establishing and protecting itself.

In Belgrade, a senior Yugoslav official accused NATO of failing to protect Serbs in Kosovo from reprisal attacks by the ethnic Albanian Kosovo Liberation Army, which has assumed a high-profile presence in the province since the departure of government forces.

"The NATO scenario is to say we will protect everyone in Kosovo, Serb and Albanian, but let the KLA demonstrate force," said Goran Matic, a minister without portfolio and a member of Yugoslav President Slobodan Milosevic's inner circle.

More than 50,000 Serbs, out of a total Serbian population of about 200,000, have fled Kosovo since the war ended. "Serb civilians can't go back to Kosovo in such conditions," Matic said.

Sniper shots rang out in Pristina shortly before Cook visited the looted offices of an ethnic Albanian newspaper and took a 10-minute afternoon walk past shuttered shops along the city's main street. "There are sniping events most days and nights" in the city, said Maj. David Pashen, a NATO spokesman. "It was nothing out of the ordinary."

After meeting with the newspaper's editor, Veton Surroi; rebel political leader Hashim Thaqi; and officials of the Serbian Orthodox Church in Kosovo, Cook remarked that "reconciliation will be much harder than the cease-fire." But he promised that the size of the peacekeeping force -- now estimated at more than 17,000 -- will double within weeks.

In Belgrade, the Milosevic government formally endorsed a proposal to end the state of war declared as NATO began its air offensive against Yugoslavia on March 24. The decision, which the federal parliament was scheduled to ratify Thursday, will permit foreign travel for military-aged men, end censorship and cancel emergency economic decrees.

The independent Group 17 research group in Belgrade estimated that the 78 days of NATO airstrikes had caused $29 billion in damage, far below estimates published by the Belgrade government. Assessing the impact of the war on the Yugoslav economy, the group said industrial production will fall by 44 percent this year, while unemployment will rise from 25 percent to 32 percent.

Correspondent Daniel Williams in Belgrade contributed to this report.

CAPTION: Marines rush to a bunker after gunmen began shooting at them at a checkpoint in southeastern Kosovo. One gunman was killed, making it the first deadly altercation involving U.S. soldiers since they arrived in the province on June 14.