ALLIED FORCES EXPECTING A TOUGH, DANGEROUS FIGHT.

Byline: Steven Lee Myers The New York Times

The three waves of NATO missile attacks and airstrikes Wednesday opened what officials said would be a protracted assault to weaken the Yugoslav military, but would do little immediately to end President Slobodan Milosevic's bloody crackdown in Kosovo.

The first barrage of several dozen cruise missiles - fired from six B-52 bombers and from four American warships, two American submarines and a British submarine - started falling on air defenses across the breadth of Yugoslavia, Pentagon and NATO officials said.

The initial wave was followed quickly by two waves of strikes by scores of fighter jets and bombers from the United States and seven other NATO nations, including Britain, Germany and the Netherlands, the officials said, speaking on condition of anonymity.

Wednesday's strikes did not target the Serbian troops and tanks that have been systematically destroying village after village in Kosovo with renewed fury since peace talks collapsed last week. Instead the battering focused largely, though not entirely, on the air defenses and the command network that pose the greatest threat to American and NATO pilots crisscrossing Yugoslavia.

``You need a few days of this,'' a NATO official said, ``before you can put in a larger number of the aircraft you need to go after the forces involved in the repression.''

The attacks on Wednesday included strikes by F-117 stealth fighters, as well as the combat debut of the B-2 stealth bomber, which at $2.1 billion apiece is the most expensive warplane ever built. Two of the B-2 bombers, flying from Whiteman Air Force Base in Missouri, each dropped 16 2,000-pound bombs.

As expected, Yugoslavia's air force and anti-aircraft missiles mounted a fierce defense, according to American and NATO officials. Secretary of Defense William Cohen said that all American and allied aircraft had returned safely to their bases after their raids, despite claims by Yugoslav officials to have downed a NATO plane.

The Yugoslav air force, made up largely of Soviet-era MiG fighters, joined the battle against NATO's warplanes, in what officials said was the most serious air-to-air combat since the Persian Gulf War in 1991. The Pentagon reported Wednesday night that NATO pilots were confident they had shot down at least three MiGs. Two of the pilots reporting shootings were Americans, the other Dutch.

Cohen and Gen. Henry Shelton, the chairman of the Joint Chiefs of Staff, declined to discuss the operation in detail but vowed to press the attack until Milosevic agreed to end his repression of Kosovo's ethnic Albanians or until his armed forces posed little threat.

``We are striking a range of military targets, including Yugoslavia's extensive air-defense system, its command and control system, and the military forces that Yugoslavia is using to suppress the Albanians in the province of Kosovo,'' Cohen said at the Pentagon on Wednesday evening.

One NATO official said that Wednesday's assault was the beginning of at least three nights of strikes. Such attacks enable American and NATO forces to build on their technological advantage, the official said. Each night of strikes will be followed by a day of work to assess damage and prepare for new raids.

Other officials said the campaign could last much longer, depending on a variety of factors, including weather and the success of Serbian defenses. The officials said they have a clear idea of how much damage NATO hopes to inflict, though how to measure it they would not say.

``We know what we want to accomplish,'' a senior Pentagon official said.

While Clinton and the Pentagon have described the military objective to be degrading the Serbian military's ability to wage war against insurgents in Kosovo, officials said they still hoped the severity of the opening shots would encourage Milosevic to relent. By day's end, however, there was no sign of that, with Yugoslavia declaring a state of war.

The Yugoslav military poses a far more formidable threat to pilots overhead than Iraq's battered air defenses ever could during four nights of American and British raids in December or during the on-again, off-again skirmishes that have continued since.

In particular, the Pentagon fears that Serbian air defenses - including 60 sets of Soviet-era SA-2, SA-3 and SA-6 surface-to-air missiles, as well as hundreds of anti-aircraft artillery and even shoulder-fired missiles that could down a low-flying jet - could pose a serious threat.

The Pentagon's planning for the raids included a prediction that at least a dozen American and NATO aircraft could be lost if the fighting continues, a senior Air Force official said. ``Given time, we can take it all down,'' the official said, ``but that comes with a high degree of risk, a risk that we as a country have not been used to in some time.''

Cohen took pains to say that the strikes are aimed at Milosevic's ``military infrastructure'' alone and that NATO forces were avoiding civilian casualties. In Brussels, NATO's Secretary-General Javier Solana tried to draw a line between the people of Yugoslavia and Milosevic's government.

``NATO is not waging war against Yugoslavia,'' he said.