U.S. Marines and a vanguard of up to 16,000 European troops stand ready to sweep into Kosovo on short notice if Yugoslav forces begin to withdraw from the embattled Serbian province as outlined in the peace deal accepted by Belgrade.

Pentagon officials involved in planning for peacekeeping said Marines from the 26th Marine Expeditionary Unit, stationed off the Yugoslav coast in the Adriatic Sea, would constitute part of an "enabling force" charged with securing Kosovo and easing the way for the return of hundreds of thousands of ethnic Albanian refugees. The Marines would pave the way for a much larger, more permanent contingent of about 50,000 U.S. and European troops.

Because the NATO troops would be entering Kosovo in the wake of a political agreement, defense officials expect little concerted opposition from Yugoslav forces. But NATO forces will be armed to deal with lingering resistance, and they are likely to confront a more challenging situation in Kosovo than they did at the start of a similar peacekeeping operation in neighboring Bosnia in 1995.

Then, a peace accord among warring Serb, Muslim and Croat groups limited the peacekeepers' tasks to traditional military functions -- monitoring the separation of factions, supervising arms caches, quelling civil disturbances. In Kosovo, the urgency of managing the return of nearly 1 million refugees who fled or were pushed out of Kosovo over the past 10 weeks will impose on NATO forces a range of civilian as well as military tasks, at least in the early months of reconstruction, officials said.

Pentagon authorities worry that a sudden, massive surge of ethnic Albanians back into the province could disrupt deployment of the international peacekeepers and overwhelm efforts to clear away mines, defuse booby traps and purify poisoned wells. But defense officials said they recognize the practical impossibility of keeping refugees back.

"It will be a very, very trying period," Gen. Wesley K. Clark, NATO's top military commander, predicted in an interview last week. "There's no government there. This is going to be very challenging."

Clark said NATO troops, working with international relief workers, will try to persuade refugees in camps in Macedonia and Albania to delay returning to Kosovo. But "this may not be possible," the general acknowledged. "This is going to be a tough, difficult, dirty job."

The initial wave of peacekeepers would move into Kosovo principally from Macedonia, where more than 13,000 European troops already have assembled, defense officials said. But other elements may come through Albania or Bulgaria, to relieve the strain on Macedonia, the officials said.

Allied forces would establish five sectors, each led by a major NATO power -- the United States, Britain, France, Italy and Germany. Just where a Russian contingent, expected to be sizable, would fit into this mix remained unclear despite weeks of negotiations. Defense Secretary William S. Cohen said "there's been no arrangement made with the Russians" as a precondition for the peace agreement.

Other Pentagon officials said they expect Moscow to accept some variant of the precedent set in Bosnia, where Russian troops have served alongside U.S. forces but report to a Russian general stationed at NATO headquarters in Belgium.

The United States has pledged to contribute 7,000 troops to the peacekeeping operation. The bulk of these soldiers would come from Army units in Germany, including three armored battalions from the 1st Infantry Division, plus groups of Army engineers, military police, artillery specialists, helicopter crews and support personnel.

Defense officials have ruled out establishing a military government in Kosovo. Instead, civil authority is to rest initially with an international group of professional administrators. But U.S. and European governments have yet to decide who will set up the provisional government -- the United Nations, which has extensive experience in this area, or the 55-nation Organization for Security and Cooperation in Europe, which had monitors in Kosovo.