In the biggest NATO attack ever, 39 warplanes from the United States and three other nations bombed an air base and nearby surface-to-air missile site used by Serbs to terrorize Bosnia.

The raid on the Udbina air base in a Serb-held section of Croatia was NATO's seventh since war broke out in Bosnia in April 1992 and by far the largest by North Atlantic Treaty Organization forces in the alliance's 45-year history.Previous NATO airstrikes have all been in Bosnia and all involved a small number of planes against highly limited Serb targets, such as a single tank.

Adm. Leighton W. Smith, Naples-based commander of NATO forces in southern Europe, said the U.S., British, French and Dutch aircraft carried out "good hits" on Udbina's runways, taxiways and on planes parked nearby.

The bombers also targeted Serb anti-aircraft batteries and one surface-to-air missile site near the airport, Smith said.

He estimated the airfield in the Serb-held Krajina area of Croatia would be out of commission for 30 days due to the damage.

Smith said in a news briefing from Naples that 39 aircraft took part in the raid: Jaguars and Mirage 2000K planes from France; F-16s from the Netherlands; Jaguars from Britain; and F-15E, F-16C and F/A 18Ds from the United States. He would not break down numbers by country.

There were no reports of losses among allied aircraft, and all returned safely to their bases even though NATO officials said Serb gunners apparently fired at the incoming planes with surface-to-air missiles.

Monday's raid was a measured response to bombing attacks by Serb planes from the Udbina air base on U.N.-designated safe areas in Bihac, 22 miles away in northwest Bosnia. U.N. commanders requested NATO air support over the weekend, but bad weather Sunday delayed the raid.

The NATO attack occurred the same day as Serbs, backed by renegade Muslim forces, were busy on the warfront: They attacked government troops across northwestern Bosnia, and U.N. peacekeepers were targeted in three separate assaults.

Two of the five other U.N.-designated safe areas - the Bosnian capital of Sarajevo and the government-held northeastern city of Tuzla - also were attacked Monday by Bosnian Serbs.

A U.N. spokesman in Zagreb said government troops in and around Velika Kladusa, north of Bihac, were under attack as well Monday by Serbs from neighboring Croatia and Muslims loyal to renegade leader Fikret Abdic.

German government spokesman Dieter Vogel called the bombardment an "inevitable response to the Serbian attacks in Bihac. We hope the Serbs will stop their attacks against U.N. protection zones."

The top U.N. official in former Yugoslavia, Yasushi Akashi, described the airstrike as a "necessary and proportionate response."

Bosnian Prime Minister Haris Silajdzic seemed pleased:

"I would like to say it is now very clear what is the character of this war in Bosnia. Some people thought that this was a civil war. It is an aggression, it is quite clear now. It is not a civil war. They are coming across the border to attack our people."

Smith said the intention of the attack "was to try to limit collateral damage. We did not want to go outside of that airfield area. We wanted also to limit the number of people on the ground who might (become) casualties as a result of the strike."

The Yugoslav news agency Tanjug said in a report from Serb-held Croatia that NATO jets struck a second time and that villages around the Udbina airfield had been hit. Tanjug said it had no word on possible casualties.

The U.N. Security Council on Saturday extended NATO air authority over neighboring Croatia after warplanes flying from the base staged airstrikes on Bosnian government targets.

The resolution specifically allowed NATO to target the Ubdina airfield, used in a napalm attack Friday on Bihac and in an air attack Saturday on the nearby town of Cazin, where nine people were killed and 15 wounded when one of two Yugoslav-made Orao attack planes crashed during a raid.

The raid on Udbina could actually aid rebel Serbs who would like to see an expansion of the war and Western retaliation, which they hope would then draw in the powerful army of Serb-dominated Yugoslavia.

U.N. troops throughout Bosnia were on high alert Monday. All nonessential U.N. personnel had been evacuated from U.N. bases in two sections of Serb-held Croatia on Sunday.

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ADDITIONAL INFORMATION

NATO bombs Serb air base in Croatia

1 Udbina: Monday, NATO warplanes bombed an air base in Serb-held Croatia used by Serb planes to attack the Bosnian "safe area" of Bihac.

2 Northwestern Bosnia: Serbs, backed by renegade Muslim forces, attacked government troops. U.N. peacekeepers were targeted in three separate assaults.

3 Sarajevo: One missile hit the roof of the Bosnian presidency and another struck the city government building nearby. Three people were injured in the attacks. Sniper fire also increased.

4 Tuzla: Government-held Tuzla was shelled Monday by Bosnian Serb forces on an hourly basis in retaliation for government troops surrounding their platoon nearby.