U.S. PLANE DOWNED; PENTAGON REPORTS AIRMAN RESCUED.

Byline: John M. Broder The New York Times

A U.S. F-117 stealth fighter went down in Yugoslavia on Saturday night, the first allied loss in the four-day Balkan conflict, even as NATO officials announced a broad new phase of the air assault on Serbian targets.

The pilot of the F-117 was rescued by a U.S. military search-and-rescue team six hours after his plane went down, according to administration officials.

Authorities in Belgrade claimed that Serbian air defenses had shot down the radar-evading stealth aircraft, one of the most advanced weapons in the U.S. arsenal. There was hostile missile activity in the area where the plane was operating, one Pentagon official said Saturday night, but there was no independent confirmation that the plane was brought down by Serbian fire.

The official Tanjug news agency in Yugoslavia said the F-117 was brought down late Saturday near Budjenovci, 35 miles northwest of Belgrade.

Although pictures of the fiery wreckage were shown on Serbian and U.S. television for hours Saturday night, administration officials did not confirm the downing of the aircraft until the pilot was safely out of Serbian airspace.

President Clinton expressed relief Saturday night that the pilot, who was not identified, was safely snatched from Serbian territory only hours after his plane went down. He said the United States and NATO were broadening the goals of the 4-day-old military operation in Yugoslavia to include attacks on equipment and troops of the Serbian army and police forces.

``As I've said from the outset,'' Clinton said in a statement released by the White House late Saturday night, ``this military operation entails real risks. However, the continued brutality and repression of the Serb forces further underscores the necessity for NATO forces to persevere.''

Reports of the loss of the U.S. aircraft stunned White House officials, who had spent most of the day consulting with European leaders about expanding the military mission in Yugoslavia.

Appalled by numerous reports of atrocities in Kosovo, including mass executions, deportations and forced marches, NATO and U.S. officials accelerated a decision to attack Serbian forces in the field, officials here and in Europe said.

Serb propaganda

The loss of the F-117 was a propaganda coup for the Serbians, who have absorbed four days of heavy pounding from U.S. and NATO bombs and missiles and the loss of at least five MiG fighters in air-to-air combat. It was the first time one of the $43 million F-117 jets has gone down in hostile action, after performing flawlessly in hundreds of missions over Iraq in 1991.

Serbian television broadcast images of the burning wreckage of an aircraft that bore the distinctive markings of the F-117 Nighthawk, with tail numbers indicating it was from the 8th Fighter Squadron at Holloman Air Force Base, N.M.

Besides the loss of the plane in Serbia, the United States faces a loss of the secret technology that allows the F-117 to evade detection. That technology includes the composite materials used for the outer surface of the plane that absorb radar instead of reflecting it back to defensive missile sites.

Any secrets lost to the Serbs could find their way to the Russians, who have been supporting Serbia.

The expansion of the allied airstrikes announced Saturday marked a new phase in the 4-day-old campaign against the Yugoslav military. Until now, airstrikes had chiefly targeted air defenses and military infrastructure, not Yugoslav troops and armor that could be used to kill ethnic Albanians in Kosovo.

`Moral imperative'

This past week, when the United States and NATO initiated the attacks, President Clinton said the West had a ``moral imperative'' to try to halt the slaughter of civilians in Kosovo. Clinton warned Slobodan Milosevic, the Yugoslav president, that if he did not cease the repression in Kosovo, NATO would ``seriously damage'' Serbia's capacity to make war.

Saturday, NATO and the United States moved to make good on that threat, setting plans for attacks by allied warplanes that would take aim at supply depots, communications facilities and troop concentrations located south of the 44th parallel in Yugoslavia. The area includes all of Kosovo and a substantial portion of Serbia south of Belgrade.

``Our aim is to strike at heavy weapons, which are of course what are used to bombard villages and kill people,'' said Jamie Shea, a NATO spokesman in Brussels, Belgium.

But officials in Washington said NATO is not yet sending low-flying combat ground-attack planes or helicopters to hit Serbian troops in Kosovo. While there are signs that NATO attacks have weakened some Serbian air defense systems, low-flying aircraft would still remain vulnerable to Serb ground fire, and the current weather in the Balkans has been too poor to permit such operations, officials said.

Shift in strategy

And while the cause of the downing of the F-117 is not yet known, allied war planners likely will treat Serbian air defenses with more respect in coming days.

The White House and NATO officials in Brussels announced the escalation of the air war and the shift in strategy Saturday afternoon after a series of conversations among President Clinton, Prime Minister Tony Blair of Britain, President Jacques Chirac of France, Chancellor Gerhard Schroeder of Germany and Prime Minister Massimo D'Alema of Italy. Later, Chirac also contacted the Russian government in an attempt to restart talks with the Serbs toward a diplomatic settlement of the conflict.

``There was total unity that the campaign was moving as planned and that it was important to move to a new phase of the air campaign, including focusing on forces in the field,'' said David Leavy, a White House spokesman.

``President Clinton believes that Slobodan Milosevic's continued repression and continued intransigence remain an obstacle to resolving this crisis and thus it was necessary to move forward in broadening the scope of the air assault,'' Leavy said. Milosevic is the president of Yugoslavia, of which Serbia is the dominant part.

A senior Clinton administration official said that the reports of Serbian violence were numerous and credible and had triggered the intensified air campaign.

``The continuing brutality made it an easy call to broaden and deepen the attack,'' the official said.

Another official said the decision to go ahead with the second phase of the air war was moved up by several days because the humanitarian disaster unfolding in Kosovo is far worse than any previous campaign against ethnic Albanians by Serbian forces.

``The Serbs have been torching villages,'' the British defense minister, George Robertson, said in London. ``There are reports that the Serbs are bombing villages to the point of obliteration. There are clear signs now that an all-out Serb offensive against the Albanian population has started.''

Robertson accused Milosevic of being a ``serial ethnic cleanser.''

Shea cited reports that large groups of Kosovo Albanians - conspicuously lacking adult men - have been spotted under the guard of Serbian forces. ``Dark things are happening,'' he said.

Despite the alarming reports and the pleas from some in Kosovo for more forceful intervention, administration officials insisted they were not planning to use U.S. or NATO ground troops to halt any killings and punish Milosevic's forces.

THE DAY`S DEVELOPMENTS AT A GLANCE

NATO expands its air assault on Yugoslavia, ordering its forces to attack tanks, artillery and troops directly involved in the government's crackdown on ethnic Albanians in Kosovo. It was the fourth consecutive day of NATO air attacks against the Balkan nation.

A U.S. stealth fighter was lost over Yugoslavia. Pentagon spokesman Ken Bacon said late Saturday that the unidentified pilot had been rescued and was safe at an ``allied air force base.'' He said the cause of the crash wasn't known and refused to provide details of the rescue.

Reports of heavy fighting and alleged Serb atrocities raise alarm about the fate of ethnic Albanians - refugees of roundups, forced expulsions and other brutal treatment. British Defense Secretary George Robertson says the Serbs were ``bombarding villages to the point of obliteration.''

President Clinton says the United States and its allies are determined to ``defuse a powder keg at the heart of Europe'' and reiterates that NATO would keep up military strikes until Yugoslav President Slobodan Milosevic agreed to a peace deal or could no longer make war.

The White House rules out the use of U.S. ground troops and NATO spokesmen say there are no plans to send troops into Kosovo.

Milosevic again rejects a peace plan, saying through an official government statement that Yugoslav citizens will not accept NATO troops based in Kosovo, as called for under the accord.

Protests against the NATO operation draw thousands of demonstrators across Europe, with 3,500 people in Stuttgart, Germany, shouting anti-American slogans and denouncing Clinton as a murderer. Demonstrations also take place in Austria, Greece, Russia, Ukraine, Norway, Italy, Romania, Cyprus and the Czech Republic.

Russian lawmakers demand an immediate halt to the airstrikes and call for upgrading Russia's military preparedness and freezing the START II nuclear arms reduction treaty.

Two Yugoslav helicopters fly over neighboring Bosnia-Herzegovina before turning back when confronted by alliance jets, NATO says.

- Associated Press 

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BOX: THE DAY`S DEVELOPMENTS AT A GLANCE (see text)