AVIANO, Italy, (AP) _ Top U.S. and NATO officials sent a strong signal Friday to Bosnia's Serbs to halt their seige of Sarajevo or face swift military action. ''We're ready to take the action that needs to be taken,'' said Secretary of State Warren Christopher.

Christopher flew to this U.S. Air Force base for a briefing from NATO Secretary General Manfred Woerner, U.S. General John Shalikashvili, the alliance's top military commander, and U.S. Admiral Jeremy Boorda, the commander of NATO air operations in Bosnia.

At a news conference, Woerner said a final decision on military plans will be taken next week. ''The military operation is ready,'' said Christopher.

''I would say to the Serbs they would be very unwise to depend upon any indecision,'' he said.

A NATO source, speaking on condition of anonymity, said the alliance was drafting a plan that envisions military action against the Bosnian Serbs but that thus far the blueprint had not reached the top military level.

The NATO military committee will meet Saturday and the alliance's governing body, the North Atlantic Council, will review the plans at a meeting Monday.

In Washington, officials at the White House and the Pentagon said they hoped the military plan would win approval at Monday's meeting but that a final decision by NATO to execute it probably would come at a later, separate meeting.

The White House said Thursday's Serb offer to withdraw forces from two strategic mountains overlooking the Bosnian capital did not go far enough. In fact, on Friday, Bosnian Serbs balked at easing the siege of Sarajevo, failing to reach agreement on handing over their strategic mountain positions to the United Nations.

Christopher stopped in Italy en route to Washington from a five-day mission to the Middle East to try to get the peace process back on track.

He met here with crews of planes that would take part in any action against Serb positions in Bosnia.

Parked on the runway were huge AWACS and C-130 flying command posts and sleek F-16 and F-18 fighters.

Marine Maj. Mark Places of Elmhurst, Ill., stood in front of his F-18D. He said he had never flown in combat but shrugged off a question on whether that made him nervous.

''It's part of the territory,'' he said. ''You constantly train for that.''

Lt. Col. Denny Rea, with the Air Force 52nd Tactical Fighter Wing, said it would take about 30 minutes to fly to Sarajevo, which is only about 250 miles from the base.

Asked about Serb air defenses, he said he assumed they had old Soviet surface-to-air missiles. But he acknowledged that they could have shoulder- fired U.S. Stinger missiles.

''They may well have stingers,'' he said. ''I don't know.''

The base at Aviano is one of several in Italy being used for various NATO operations in Bosnia, including enforcement for the no-fly zone and support for the economic and arms embargoes.

Also involved is the USS Theodore Roosevelt, an aircraft carrier stationed in the Adriatic.

Christopher said his stop at the base was ''a reminder it is now time for the Serbs to stop the strangulation of Sarajevo and other cities in Bosnia.''

He said it was ''time for the Serbs to take the action they need to take to avoid the military action being contemplated here.''

Christopher brushed off statements by Bosnian Serbs that they are ready to lift the seige of Sarajevo, saying that ''we simply cannot accept empty promises as a cover for continued aggression.''