SARAJEVO, Bosnia-Herzegovina (AP) _ A U.S. fighter jet was shot down today by a missile in Bosnia, NATO said. A search-and-rescue operation was under way for the pilot of the F-16C aircraft, who an Italian official said survived.

The downing of the single-seat Fighting Falcon came as the West was hardening its stance toward the Bosnian Serbs, adding Allied forces within striking distance of rebel Serbs who had taken 377 U.N. personnel hostage, precipitating a weeklong crisis.

As the rescue mission for the downed pilot went on, Bosnian Serb leaders appeared to be backing off a promise to the Red Cross earlier today that they would free the hostages. By nightfall, no releases had occurred.

NATO said the U.S. jet was downed about 3 p.m. today (9 a.m. EDT) while flying as part of NATO air patrols enforcing a ``no-fly zone'' over Bosnia.

A Bosnian Serb military source speaking on condition he not be named told The Associated Press that the fighter jet ``exploded in midair'' near Mrkonjic Grad, about 25 miles south of Banja Luka in an area controlled by Serb rebels.

A second Bosnian Serb military source said a parachute was seen shortly before the plane crashed on the outskirts of Mrkonjic Grad.

President Clinton said he was ``very concerned'' about the incident and the pilot's fate but declined to elaborate due to the sensitivity of the matter and ongoing rescue operations.

The downed craft was one of two NATO planes flying together in a reconnaissance operation high over northern Bosnia in an area controlled by hardline rebels.

A NATO source said a sophisticated Russian-made SAM-6 surface-to-air missile known to be in the rebels' arsenal especially in that region downed the aircraft.

In Washington, a U.S. defense official said he could provide no details about the rescue operation without compromising the mission. He said helicopters are involved, with jet fighters providing cover.

Italian Foreign Minister Susanna Agnelli said without elaborating that the United States had informed European leaders the American pilot was alive.

NATO planes patrol Bosnian skies in connection with Allied attempts to end the war that began in April 1992 when Bosnia seceded from Yugoslavia. About 200,000 people have died in fighting since or disappeared.

The last time a NATO plane was downed was April 16, 1994, when a British Sea Harrier was shot down near Goradze while attempting an airstrike on Bosnian Serb positions. The pilot was rescued from government-held territory by Bosnian soldiers.

NATO spokesmen in Naples, Italy, said the American plane that was shot down was part of the U.S. force based at Aviano air base in northeast Italy.

``The shooting down (of the NATO craft) ... was clearly a provocative act,'' said Muhamed Sacirbey, the Bosnian foreign minister at U.N. headquarters in New York.

The NATO plane went down less than two hours after international Red Cross officials announced they had been assured by the Bosnian Serb political leadership that all of the nearly 380 U.N. hostages would be freed by Saturday.

About 120 U.N. peacekeepers were to be bused to Yugoslavia via Zvornik, on the Bosnian border, this afternoon, other Pale sources said. But that did not occur.

Nikola Koljevic, a relatively moderate Bosnian Serb leader, denied he ever ``told anyone that the imprisoned UNPROFOR personnel would be released unconditionally, as it was reported by some Red Cross officials,'' Belgrade's independent Beta news agency reported.

UNPROFOR is the name of the U.N. mission in Bosnia.

Andreas Pfeffner, head of the International Committee of the Red Cross in Pale, told the AP he was ``still waiting for official confirmation from the Serb authorities. The ICRC at the moment has no indication leading towards the realization of the release.''

Today's developments followed increasing military pressure by the West for the unconditional release of the peacekeepers, taken in retaliation for NATO airstrikes last Thursday and Friday on ammunition depots outside Pale. The prospects for full release seemed high earlier in the day.

``From the highest authority in Pale we have been informed that either today or by tomorrow all UNPROFOR personnel would be released,'' announced Lucie Sternthal, deputy chief of the Red Cross mission in Pale.

Only hours earlier, Serb soldiers kidnapped three Ukrainian peacekeepers from an observation post near Gorazde, bringing to 377 the number of U.N. soldiers held hostage or blockaded in Bosnia.

Bosnian Serb leader Radovan Karadzic warned Thursday against trying to rescue the U.N. hostages, scattered throughout Serb-held territory, saying such attempts would lead to slaughter ``that would resemble a butcher's shop.''

He said the hostages won't be released unless NATO pledges not to launch any more airstrikes on Bosnian Serb territory.

A Swedish U.N. civilian official, however, was released after being held briefly by Serb rebels in northern Bosnia.

Karadzic repeated demands for the complete demilitarization of Sarajevo and other government-held enclaves and a halt to clandestine arms shipments to the Bosnian army.

But Western allies instead flexed their military muscle, pinning their hopes on a new rapid-deployment force that could either reinforce the U.N. mission in Bosnia _ or help it leave.

Britain began dispatching the first of a threatened 6,000 new troops to Bosnia, while France and the United States positioned crack military units in the Adriatic in a show of force.

Clinton in a U.S. policy reversal this week said he could make U.S. ground troops available to help move U.N. peacekeepers to safer posts in former Yugoslavia.

Formation of the new force was expected to dominate a Saturday meeting of European defense chiefs and military commanders in Paris.