The Pentagon plans to have a full force of 20,000 American peacekeeping troops deployed in Bosnia by February in an operation that will cost about $2 billion, Defense Secretary William Perry told Congress Thursday.

He said U.S. and other members of the international force could come under attack from gangs or individuals opposed to the accord brokered by the United States. And, Perry testified, the NATO force of 60,000 faces risks of harsh weather and mines.But the Americans were well-trained, he said, even using a model of a Bosnian village to prepare for sniping and other contingencies. "The units are run through every conceivable scenario we could think up," he told the House International Relations Committee.

Perry said the cost of the operation was estimated at $2 billion, with $1.2 million for the troops in Bosnia and $500 million for support units in the region. The remaining $300 million already has been appropriated for missions that include flying air operations out of Italy.

Some committee members responded with skepticism but with none of the harsh criticism directed at the administration only a few weeks ago.

"Except for second-guessing, the arguments are largely over," Rep. Jim Leach, R-Iowa, said quietly even though he said the administration had not provided an "exit strategy" apart from an approximate date to end the deployment.

Perry, Secretary of State Warren Christopher and Gen. John Shalikashvili, chairman of the Joint Chiefs of Staff, went before Congress to seek support that Pres-ident Clinton says he does not legally need for the deployment. Without NATO's intervention, Perry said, the war that claimed an estimated 250,000 lives would erupt again.

"The United States is the only nation that can lead that force to success," he said.

"The mission will be tough, and there's no doubt about it, and we have to be prepared for casualties," Shalikashvili said.

The agreement to end the 43-month ethnic conflict is due to be signed in Paris Dec. 14. Bosnian Serb leaders have asserted the agreement could turn the former Yugoslav republic into another Lebanon. They want to renegotiate a clause in the agreement, initialed last weak at Wright-Patterson Air Force Base in Ohio, so they could retain control of Serb-inhabited suburbs of Sarajevo, the capital.

The Clinton administration has ruled out any revisions in the settlement reached by Bosnia, Serbia and Croatia. However, Perry acknowledged, "there will be risks in this operation," not from the three governments but from individuals or gangs challenging NATO'sauthority to intervene.

Addressing the House committee from a lectern, and using a pointer and placards, Perry detailed the deployment schedule, saying it had been based on a one-year timetable.

An advance NATO detachment, including some U.S. troops, will go to Bosnia-Herzegovina in the next few days, Perry said. After the accord is signed, combat units, including the U.S. Army's 1st Armored Division, will move in from Germany by train, and the buildup will be rapid, Perry said.

By the end of the second month, he said, "we will have our entire force in." Perry also assured Congress everything that can minimize risks to the troops will be done.

The troops will be under the command of Gen. George A. Joul-wan, an American who is NATO's supreme commander. They will maintain a 21/2-mile-wide zone of separation between the foes and monitor the withdrawal of warring factions into boundaries agreed to in the Ohio accord.

Perry said U.S. forces will not train and equip the Bosnian army. But, he said, the United States would favor building up the Muslim-led army only if the effort to reduce arms on all sides of the dispute fails.

Perry said the NATO force should be able to complete its military tasks within six months and then leave six months later with a promising new stability in place.

These tasks, as spelled out in the Dayton accord, include monitoring the separation of rival forces, ensuring that heavy weapons are placed in designated areas and policing the withdrawal of forces from lands to be transferred from one faction to another.

Perry said the peacekeepers would spend the second six months providing a "security environment" so that longer-running tasks to be carried out by civilian agencies, such as rebuilding Bosnia's infrastructure and reconstructing its economy, can get off to a good start.

Meantime, the White House announced that Clinton will attend the Dec. 14 signing ceremony in Paris.