Western specialists said today they believe Israel used a super weapon still in the experimental stage in the United States to knock out dozens of Syrian tanks earlier this month in the Bekaa Valley of eastern Lebanon.

The specialists, who saw dozens of the destroyed tanks, said the evidence indicates that the Israelis employed a weapon resembling the American Sadarm--for sense and destroy armor--still in the test stage in the United States.

Sadarm is part of a family of anti-armor weapons being developed under a Pentagon program called Assault Breaker. But defense officials in the United States said they have not yet perfected the weapon for American use nor sent it to Israel or any other country. However, they acknowledged that the description given of the Israeli armor killer in action sounded like the way the American Sadarm is supposed to work.

The Western specialists, piecing together the evidence, said the Israelis apparently fired their version of Sadarm from an American-made, eight-inch, self-propelled howitzer. The shell seemed to break into three parts in the air, and then each one steered into the cupola of a tank where the armor is thinnest and thus easiest to pierce.

It is also possible that the tank-killing weapon was dropped from an airplane and only seen after a parachute opened over the Syrian tank formations. A canister carrying the tank-destroying projectiles could have been deployed in this fashion, weapons specialists said, if Israel--like the United States--is having problems with firing the new weapon out of artillery tubes.

Once over a tank formation, each of the projectiles is supposed to pick out its own target. One method is for it to home in on the heat of the tank, while another is to use onboard television. The Pentagon is said to be experimenting with both kinds of guidance.

Specialists here noted that the dozens of knocked-out Syrian tanks in the Bekaa north of the Beirut-Damascus highway looked as if they had been struck in the same vulnerable cupola area atop the turret. This would suggest television guidance or some technique other than heat seekers, which would have drawn the projectiles into the rear engine area of the tanks.

Several groups of tanks, each containing scores of them bearing the tell-tale marks of destruction by a Sadarm-type projectile, are still visible through field glasses from the peaks of the Mount Lebanon range, according to the specialists analyzing the weapons and tactics used against Syria's Soviet-supplied Army.

The idea of the Pentagon's Assault Breaker weaponry is to stop a whole group of tanks at once, regardless of whether they are moving in typical formations--four in front and one in the rear--or fanned across a broad landscape, officials said. The guidance on a Sadarm projectile focuses on only a limited patch of ground to avoid the bunching of the weapons on a single tank.

Pentagon officials, when told about evidence suggesting Israel had used a super weapon like Sadarm in battle, said it was possible that the Israelis had gone beyond the United States in perfecting that type of weapon. If this turns out to be the case, officials said they would consider it a stunning technical achievement, given the problems that have plagued the American development of Sadarm and anti-armor weapons like it.

Lebanese military sources were quoted as saying that the new weapon's accuracy so frightened Syrian tank crews that they abandoned their armor in the Bekaa and deserted the battlefield.