WASHINGTON (AP) _ NATO's threat to send warplanes over Sarajevo is limited to halting Serbian shelling of the Bosnian capital and won't risk involving U.S. forces in an open-ended quagmire, Pentagon officials say.

The NATO operation is a ''set, definite and precise mission, that is to reduce the carnage caused by the shelling or the potential to shell Sarajevo,'' Brig. Gen. James Hill said at a Pentagon briefing Thursday. ''That's the only objective we have at this stage.''

''It does not start us on a slippery slope toward a generalized campaign of air strikes,'' he said.

Should Serb shelling resume, NATO war planes would strike at a range of targets, not just those inside a designated security zone around Sarajevo.

''If there is shelling from outside (the 13-mile zone), there are provisions for responses to that,'' Walter Slocombe, the deputy undersecretary of defense for policy said at the briefing.

Nor will NATO jets confine retaliatory strikes to a specific battery or mortar, Slocombe said.

''If there is an attack, we will not put ourselves in the position of having to find the particular gun that fired the particular shell,'' the official said.

NATO aircraft have been authorized to destroy both heavy weapons and ''military-related'' targets, he said. Those categories presumably could include such items as ammunition dumps or supply depots, but Slocombe declined to be specific, saying such information could be of use to an opponent.

More than 100 fighter-bombers from the United States and NATO nations such as France and Britain could take part in the strike force.

The aircraft have been taking part in Operation Deny Flight, patrolling Bosnian air space since April from air bases in Italy and Navy warships in the Adriatic.

Under an ultimatum issued by the NATO allies on Wednesday, the Serbs have 10 days to move their heavy weapons 13 miles from central Sarajevo or place them under U.N. control. Otherwise, they will be subject to NATO air strikes.

Hill, from the Joint Staff's office of strategic plans and policy, said the 13-mile distance was chosen because it covers the range for most of the weaponry used by the Serbs.

Also, a larger region would have increased the monitoring area - and burden - for U.N. forces on the ground and aircraft overhead, the general told reporters.

The Serbs are believed to have about 200 to 300 artillery pieces or large- caliber mortars, 50 tanks, various antiaircraft artillery weapons and some shoulder-fired surface-to-air missiles inside the proposed banned region, Hill said.

The operation is not without risk, and to find and detect any such weaponry ''is not an easy task,'' Hill said. But because of the Deny Flight Operation, he said, ''our pilots are well-trained. ... We've had plenty of experience in the area.''