SARAJEVO -- In NATO's largest attack ever, 39 warplanes Monday bombed a rebel Serb airport in Croatia that was used to launch cross-border raids on shrinking Bosnian government holdings in the northwestern part of the country.


The air strike, by jets from the United States, Britain, France and the Netherlands, was the seventh since the start of the Bosnian war and expanded NATO's involvement in combat in the conflict in former Yugoslavia to outside the borders of Bosnia for the first time.


Besides bombing the airfield at Udbina, Croatia, 35 kilometers southwest of the Bosnian town of Bihac, the planes also targeted Serb anti-aircraft batteries and one surface-to-air missile site near the airport, said Admiral Leighton Smith, NATO commander for southern Europe.


"Initial reports are that the strike was successful," the U.S. admiral said. Although Serb surface-to-air missiles apparently had been fired at the NATO planes, all the planes returned safely to their bases.


The UN commander for former Yugoslavia, General Bertrand de Lapresle, specifically requested that the strike should be confined to runways and that NATO should not destroy aircraft, Smith told reporters in Naples, Italy.


"Our intention was to try to limit collateral damage," Smith said in a television interview. "We did not want to go outside of that airfield area, and we wanted to limit the number of people on the ground who might be casualties as a result of the strike."


Croatian Serb military officials said one person died in the air strike and several were wounded. There was no independent confirmation and no NATO comment on Serb claims that civilian targets, including homes, were targeted.


A UN spokesman in Sarajevo, Major Herv? Gourmelon, said some Czech peacekeepers, who were posted near Udbina, had been taken hostage after the raid. He did not provide further details.


Smith said the raid was a warning and sought to put the field out of use for around 30 days -- "not out of commission for an awfully long time. That was not our intent."


The air strikes actually may have played into Bosnian and Croat Serb hands. Their territories are contiguous and they are attacking jointly because they know they must stick together to survive.


An escalating cross-border conflict might put pressure on Serbian President Slobodan Milosevic to help his Serb brethren.


The powerful Serbian leader cut off most aid to Bosnia's Serbs in August in exchange for eased international sanctions on Serbian-dominated Yugoslavia.


He continues to wield influence in Serb-held parts of Croatia and appeared ready to mediate.


Senior UN envoy Yasushi Akashi told reporters Monday that he and Milosevic would meet Wednesday with Milan Martic, head of Croatia's breakaway Serbs.


Martic condemned the bombing as "an insolent and vandal attack ... which we haven't provoked at all."


But Akashi, the top UN official in former Yugoslavia, insisted the NATO air strike was a "necessary and proportionate response."


In Belgrade, the Yugoslav government condemned the strike as "unfounded and irresponsible."


Serb planes flying from Udbina have bombed the Bihac area three times since Nov. 9, including one attack Friday on Bihac city with napalm and cluster bombs. On Saturday, nine people were killed and 15 wounded in nearby Cazin when one of two Yugoslav-made Orao attack planes crashed during a raid.


"I hope that today's NATO air strike will deter any further attack on the Bihac 'safe area' and its surroundings" or on UN personnel within Bihac, Akashi said.


There was no immediate word on any effects the air strikes had on fighting in the Bihac area.