ZAGREB, CROATIA, NOV. 21 -- NATO fighter-bombers launched the biggest air raid in the history of the alliance today, dispatching more than 30 warplanes to bomb antiaircraft weapons and the runway at a rebel Serb airfield in Croatia.

The allies refrained from destroying Serb planes on the ground, and the U.S. admiral commanding the operation said the runway could be functioning again soon. But he added that the point of the attack was less to destroy equipment than to warn Bosnian Serb leaders against repeating recent air attacks launched from the airfield at Udbina, about 20 miles inside Croatia.

The NATO attack was triggered by two consecutive days of Serb airstrikes by planes based at Udbina on the Muslim enclave of Bihac in northwestern Bosnia.

While it was the boldest show of international force since Yugoslavia's wars of secession began four years ago, the 45-minute assault by U.S., British, French and Dutch planes did not seek to pulverize the airfield or blow up planes.

U.S. Adm. Leighton Smith, the commander of NATO's southern region, said, "Our purpose today was to send a signal, and I believe we did so."

He said the raid was not of sufficient magnitude to put the runway at Udbina completely out of action. "It's fairly easy to fill up a hole in an airfield, so I don't expect this airfield to be out of commission for an awfully long time," he told reporters in Naples.

Smith said that after consultation with the U.N. commander, French Lt. Gen. Bertrand de Lapresle, "we chose not to strike the aircraft because they're all spread out in separate areas... ." He said they wanted to limit casualties from the strike.

A senior military officer at the Pentagon said an estimated 15 to 20 Serb Orao warplanes were based at the airfield. He said there was no reported damage to any of the allied aircraft. Asked about Serb casualties, the officer said he could not tell from the battle reports if there were any.

The Yugoslav news agency Tanjug said that one person was believed killed and an unspecified number of civilians wounded in the attack, the Reuter news service reported. Tanjug said its information came from the Krajina Serb army, but there was no independent confirmation.

While impressive in form, the attack was moderate in nature, reflecting two main issues that have dogged the U.N. operation in the former Yugoslav republics for more than two years -- unwillingness to take sides in the war and its reluctance to confront the Serbs.

"We are in a very sensitive and delicate situation," explained Yasushi Akashi, the chief of the U.N. mission here, which is known as UNPROFOR. "If we do not act, UNPROFOR will be considered incompetent and spineless. If we act too vigorously and aggressively, we may create a situation in which there is escalation leading to most tragic consequences. We tried to tread this narrow path."

President Clinton called the strike "the right thing to do" and "a strong and entirely appropriate response."

It marked the fifth time Western warplanes have hit Serb targets since Feb. 9, when the alliance ordered the Bosnian Serbs to withdraw their heavy weapons from around the capital of Sarajevo.

Akashi said he would meet Wednesday in Belgrade with Serbian President Slobodan Milosevic and the leader of the Croatian Serbs, Milan Martic, in an attempt to calm the situation and persuade the Croatian Serbs to stop their attacks across an internationally recognized border and lift their blockade of food and medicine going into Bosnia's Bihac pocket.

Fighting continued today in Bihac. Croatian Serbs are supporting an offensive into the pocket from the north and west by rebel Muslim fighters loyal to renegade Muslim businessman Fikret Abdic. Meanwhile, Bosnian Serbs are pressing from the east and south.

Martic, whose forces have been attacking Bihac for weeks, condemned the air raid as "an impudent and vandalistic attack" and said it would not contribute to peace in the region. However, Akashi quoted the president of the self-styled Krajina republic as saying "he would do his utmost to restrain any emotional reaction" by his troops.

Indeed, the question now is what effect the airstrike will have on the increasingly volatile security situation throughout Bosnia and Croatia and how will it affect relations among Serb leaders, locked in an apparent power struggle stretching from Belgrade through their nationalist outposts in Bosnia's Pale and Croatia's Knin.

The main risk of the early afternoon strike over clear skies on the Udbina airfield 40 miles east of Croatia's Adriatic Coast was political, not military. Although Serb fighters peppered the sky with antiaircraft rounds, all the jets, including U.S. Air Force F-16s, F-15Es, radar suppressing EF-111s and Marine F/A-18Ds, returned safely to bases in Italy.

The operation, which was launched about 11:30 a.m. (5:30 a.m. EST), left five "major craters" in the airport's runway at taxiway intersections and one "direct hit" on a taxiway "with a couple of near misses," according to the senior military officer at the Pentagon.

He said the Serb antiaircraft defenses around the airfield were bombed.

Between 30 and 40 allied aircraft were involved, about two-thirds of them U.S. planes, the Pentagon officials said. The jet fighters and reconnaissance aircraft flew out of five bases in Italy and dropped a variety of bombs, tailoring the type of munition to the target.

Cluster bombs were used to take out a surface-to-air missile site at the north end of the airfield; precision-guided bombs were dropped on the runway and nearby anti-aircraft batteries; and nonprecision bombs were let go on the taxiways, according to the Pentagon.

U.N. officials said a planned strike Sunday had been called off because of bad weather. What concerned U.N. officials, however, was the possibility that NATO's strike could spark a wider war in the Balkans by prompting the Croatian Serbs to attack Croatia, which would then jump into the fray. Martic threatened Croatia over the weekend after Croatian President Franjo Tudjman on Friday approved NATO's request to hit Serb targets on Croatian soil.

U.N. officials said they also feared for the lives of the 43,000 lightly armed peacekeepers here, especially the 1,200 Bangladeshi troops living in the middle of a war zone in the Bihac pocket. U.N. officials reported that Serb troops held two Czech soldiers manning a position near Udbina airbase for several hours today but released them in the afternoon.

Another question about the airstrike is how it will change relations among the Serbs.

In August, Serbia's Milosevic declared he was shutting Yugoslavia's border with Bosnia to punish the Bosnian Serbs for not signing an international peace deal. Since then, the Bosnian Serbs have placed the Croatian Serbs under a great deal of pressure to help with their war effort.

When Bosnian Muslim forces burst out of Bihac pocket last month, grabbing 95 square miles in their biggest gain in the 31-month-old war, that pressure increased. Croatian Serb troops gave weapons, training and transport to thousands of rebel Muslim fighters who had fled Bihac in August after the crushing of an uprising by forces loyal to Sarajevo. Soon Croatian Serb forces began using tanks and artillery to blast the enclave.

On Nov. 9, Serb planes took off from Ubdina, in the first of three airstrikes on the pocket, which included the use of napalm and cluster bombs and a deadly assault on an apartment block filled with refugees.

U.N. officials speculated that radical Serb nationalists want to see a widening of the war to include the Croatian Serbs. Such a development -- in which an alliance of Croatian and Bosnian Serbs would take on both Bosnia's Muslim-Croat federation and Croatia itself -- would pose a serious challenge to Milosevic, who would be hard pressed to continue his blockade of the Bosnian Serbs and his support of Bosnia's peace plan.

Staff writer Bradley Graham contributed to this report from Washington.