U.S. Troops Key to Policing Truce : Bosnia: Sweeping peacekeeping plan developed with NATO allies calls for rapid occupation of air bases and seaports by American forces if peace plan takes hold.

The United States and its allies in the North Atlantic Treaty Organization have developed a sweeping new peacekeeping plan for Bosnia-Herzegovina that envisions rapid occupation of key air bases and seaports by U.S. Marines and paratroopers, followed by deployment of 12,000 American combat troops.

The plan, still to be approved formally by President Clinton, is designed to help enforce the pending peace plan, assuming the Bosnian Serb parliament ratifies it on Wednesday.

If the Serbs reject the plan, Clinton has threatened military action, including U.S. air strikes, against Serbian forces.

The peacekeeping proposal calls for a quick movement by U.S. Marines into Adriatic seaports while paratroopers secure Sarajevo’s airport, which then would be used as a staging area for an airlift of personnel and supplies. The 1st Armored Division, based in Germany, would provide the combat troops.

No U.S. troops would be sent until the Bosnian Serbs demonstrated that they had accepted the peace accord. After the initial military operations, the force would be expanded to between 60,000 and 75,000 multinational troops to secure the country militarily and begin improving roads and bridges and restoring civilian government. The United States would provide about one-third of the total force.

Military sources confirmed that a small number of U.S. special operations forces already have entered Bosnia in an effort to pave the way for possible air strikes by pinpointing possible targets and to scout out logistics.

There was no immediate indication how quickly NATO is prepared to move if the Bosnian Serbs accept the peace plan. If the Serbian parliament rejects the plan, the United States may be prepared to begin launching air strikes within a few days.

Hopes that the Bosnian Serb parliament might accept the peace settlement grew stronger Monday as shelling by guerrillas in Bosnia tapered off some. But Serbian leaders were reported to be sharply split, with some angry that Bosnian Serb leader Radovan Karadzic signed the pact last weekend.

Clinton on Monday underscored his commitment to send U.S. forces to a peacekeeping mission, telling reporters at the White House: “We said several weeks ago that the United States would be prepared to support a United Nations effort, heavily engaged in by Europeans, to help enforce a peace if a peace was made.”

If the NATO plan for a peacekeeping force is carried out, it would be the largest--and riskiest--such operation in U.N. history. By contrast, the peacekeeping force now in Somalia totals about 25,000 troops. It was preceded by six months of U.S. military operations there.

Sources familiar with the NATO plan said that the force would be composed mainly of light infantry units, with some mechanized units, organized for separate but coordinated intervention and rapid-response missions, as well as logistic and administrative support.

Once the U.S. troops completed the initial takeover, the force would be broadened to include troops from Britain, France, Spain, Canada and possibly--for the first time--Russia. Although some of the American troops would come from the United States, most would be drawn from U.S. bases in Europe.

U.S. officials said that the bulk of the 12,000 combat troops and tanks slated for the early stages of the operation would be supplied by the 1st Armored Division, based at Bad Kreuznach, Germany, and the 325th Airborne Infantry Regiment at Vicenza, Italy.

The Marines most likely would come from the amphibious ready group on the Saipan, which carries about 2,200 Marines from Camp Lejeune, N.C.

Pentagon officials said that, once the force was expanded, the United States might have up to 50,000 troops assigned to the peacekeeping operation--20,000 or so stationed in Bosnia and 30,000 at sea, manning supply operations at home or waiting to relieve the combat forces.

However, U.S. officials cautioned that many key issues remain undecided, ranging from which country will actually be asked to command the operation to the precise rules of engagement--that is, under what circumstances the troops would be allowed to fire.

U.S. officials have insisted that the most effective way to secure the peace is to allow the troops to fire at will--and to punish those who fire at them--as they have been permitted to do in Somalia.

There also are major questions about how much control the Bosnian Serbs’ self-styled government would have over the widely scattered Serbian guerrilla units if it agreed to the accord.

“There are so many places this plan can go wrong, and it will be such a decentralized operation that almost anything can happen,” said Don Snider, military analyst at the Center for Strategic and International Studies in Washington.

Snider noted that the latest version of the peace agreement, which was negotiated by U.N. envoy Cyrus R. Vance of the United States and European Community representative Lord Owen, would divide Bosnia into 10 largely autonomous provinces, linked by a network of corridors.

“You just add up the amount of boundaries you’ve got there and it’s hard to say how to enforce it,” he said.

Bosnia’s mountainous terrain and limited physical infrastructure will make the operation even more difficult for NATO troops. For instance, the country has no huge, modern air fields, port facilities or underground storage, as Saudi Arabia offered allies during the Persian Gulf War.

Still, sometimes such efforts work surprisingly well. Retired Gen. Edward C. Meyer, former Army chief of staff, pointed out that the allies deployed a similar force to police the Israeli-Egyptian border in 1976 and that it has operated successfully, almost without incident.

Meyer said that the effort in Bosnia would use vastly improved technology, from spy satellites to helicopters and “non-lethal” devices used by special forces--chemicals that make roads impassable or turn ammunition into duds.

Times staff writer Melissa Healy contributed to this article.