From the minute they arrive in Kosovo, NATO troops will become the police, town council and public works department until the messy problem of creating a local security force and a provisional government is worked out.

To run what will be essentially a military government, NATO peacekeepers will have unlimited powers to control and govern the battered Serbian province. Their commander will have the authority "to do all that he judges necessary and proper, including the use of military force," to protect his troops and to allow them to carry out their duties, according to an agreement signed yesterday by NATO and Yugoslav officers.

Simultaneously, the peacekeeping force will be responsible for monitoring the movement of 40,000 heavily armed Yugoslav troops from Kosovo northward to other parts of Serbia, while the first of some 850,000 refugees seek to return from Albania to the west and Macedonia to the south. Eventually, the troops will have to find and disarm miles of land mines and rebuild dozens of recently bombed bridges and roads.

Their reconstruction will make it possible for 50,000 troops, a NATO-led peacekeeping contingent called KFOR, to move in and stay for the foreseeable future. Of these, 7,000 will be Americans.

They will do all this while the well-armed but poorly managed Kosovo Liberation Army, the rebel force that continued to fight Yugoslav forces even yesterday, is expected to try to reassert control over local life in many villages as the Yugoslav army pulls back. In addition, thousands of Yugoslav soldiers and police will still blanket the province -- although they are slated to withdraw -- when the first peacekeeping elements move in.

"Military men are going to be challenged to perform tasks unlike any they have ever faced before," said Sen. John W. Warner (R-Va.), chairman of the Senate Armed Services Committee.

Pentagon officials predicted that the first peacekeeping forces will enter Kosovo immediately after a U.N. Security Council resolution authorizing their deployment, expected sometime today.

The Pentagon reported that 1,900 Marines, who had been waiting aboard three ships off Greece, landed on the beach of Litohoro early today and prepared to move to Macedonia, getting into position for a move into Kosovo. At the same time, an Army headquarters element from Germany will start arriving in Macedonia aboard C-17 transport planes and other Army units -- AH-64A Apache helicopters, tanks, artillery and 1,700 troops -- will be entering Macedonia overland from Albania.

The Kosovo Liberation Army, the main guerrilla force that fought the Serb forces until yesterday, was not party to the NATO-Yugoslav agreement and is not mentioned in the text. NATO and U.S. officials are hoping they can turn the young, brash leaders of the rebel force into a new 3,000-strong police force and that the loose-knit group will be able to impose discipline on factions that do not support NATO's goals because they do not include an independent Kosovo.

KLA leaders, who have at least several thousand guerrillas inside the province, say they will abide by the spirit of the agreement by limiting movements and have vowed not to pursue retreating Yugoslav forces. Both the KLA and NATO officials said they hope to complete within weeks an agreement on the particular types and quantities of weapons the secessionist rebels are to turn in.

NATO is seeking to disarm the KLA of heavy weapons but does not expect the rebels to turn in the abundant supply of rifles and machine guns they were able to buy from neighboring Albania and on the black market. Shinasi Rami, a spokesman for a coalition that includes the KLA, said that imposing a quick deadline for turning in weapons "would put Kosovo society in extreme strains." Rather, he said, "it would be a wise move to keep them integrated in the structure" of a new Kosovo.

Many U.S. officials worry that the rebel force will seek revenge against the Serb civilian population, will turn increasingly to criminal activity and will, once the Serb troops are gone, try to undermine the development of civilian political rule and seek instead to become a dominant political force.

"The KLA will keep its options open," said Ivo Daalder, a former National Security Council member and Balkans expert. "That's going to be the number one problem."

Under the agreement, Yugoslav troops will withdraw in three phases from Kosovo.

First, Yugoslav troops in northern Kosovo must leave by the end of today and must pass through one of four border crossings that NATO will monitor. Over the same period, all Serb aircraft must cease flying, and all air defenses -- including radars and surface-to-air missile batteries -- must cease operations.

By the third day, all air defense systems must be moved 15 miles north of the Kosovo border.

By the sixth day, all Serb troops in the southernmost zone along the Macedonia border will have to move north. Three days later, the central zone must also be vacated of troops, and 11 days after the agreement all Yugoslav forces must be out of the province.

The agreement also sets up a three-mile demilitarized zone along Kosovo's provincial borders with the rest of Serbia.

The Yugoslav government is required to turn over, within two days, detailed maps recording location of land mines and other booby traps that NATO believes will endanger troops and returning refugees.

One of the major points left unresolved is how many Yugoslav forces will be allowed back into Kosovo. While NATO authorities have acceded to Belgrade's demands for some presence to patrol the border and maintain historical and cultural sites, the exact number is to be worked out in a separate accord later. The agreement reached last week with the Yugoslav leader, President Slobodan Milosevic, specified the final number would be "hundreds, not thousands."

In at least one respect, defense officials said the peacekeeping operation in Kosovo should be less risky than the one that began in Bosnia 3 1/2 years ago.

"In Bosnia, we were going into a situation where we actually had to separate the forces and set up and patrol zones of separation between the Muslim and Serb sides," said Pentagon spokesman Kenneth Bacon. "Here, the Serbs are supposed to be out."

U.N. Balkans envoy Carl Bildt, however, said Europe and the United States will have to "take Kosovo from virtually nothing to practically everything in the next few years. This is to be . . . the most complex peace implementation operation ever undertaken by the international community in modern times."

Peacekeepers on the Move

The force will consist of 50,000 troops -- 17,500 of whom are now assembled in Macedonia, the staging area.

Commander: Lt. Gen. Mike Jackson

* Six nations -- U.S., Britain, France, Germany, Italy, Russia -- have committed troops, and several others have pledged to contribute troops. They will be deployed in designated sectors.

U.S. contingent:

Commander: Brig. Gen. John Craddock

* 1,700 U.S. Army soldiers yesterday began moving by road from Albania to Macedonia.

* 1,900 Marines, of the 26th Marine Expeditionary Unit, now aboard ships off the coast of Greece are to move ashore in Thessaloniki and move by road to Macedonia.

* 7,000 U.S. Army soldiers to deploy from Germany. An advance party of 200 Army troops from Germany will be among the first U.S. forces into Kosovo.

The Kosovo peace deal provides for a NATO-led international peacekeeping force to be deployed once Yugoslav forces have begun to pull out and NATO has suspended airstrikes.

SOURCE: U.S. Defense Department