A Czechoslovak military jet crossed into West Germany Saturday and fired at least two missiles at a U.S. Army helicopter flying a "routine" surveillance mission near Czechoslovakia's southwest border, Pentagon officials said yesterday.

The Cobra attack helicopter, carrying a two-man crew, was not struck and returned safely to its base near Nuremberg without returning the fire, said Defense Department spokesman Robert B. Sims.

The United States filed a "strong protest" with the Czechoslovak Embassy here Monday, Sims said. He told reporters at a briefing that the Cobra did nothing to provoke "this irresponsible act which endangered the lives of the U.S. crewmen."

The incident was the 17th violation of West German airspace by Warsaw Pact aircraft in the last six months but the first in which missiles were fired at a U.S. aircraft, according to Sims.

In April 1984, a Cobra came under missile and cannon attack from two Soviet-built warplanes, flown by Czechoslovaks, as it patrolled the border between West Germany and Czechoslovakia. The Pentagon acknowledged later that the U.S. helicopter, which escaped damage, had strayed more than six miles into Czechoslovak airspace.

A Pentagon official said Saturday's attack took place near the West German town of Freyung about a mile from the Czechoslovak border. He said intelligence reports indicate that the pilot of the Czechoslovak L39 jet trainer knew the Cobra had not strayed across the border.

"You have to believe either they're not under positive control or the incident was deliberate provocation," he said of the Czechoslovak pilot.

A State Department official said the attack reflected a "cat and mouse game" played by opposing aircraft patrolling the borders that separate Warsaw Pact and North Atlantic Treaty Organization nations. He said there is no indication that the L39 intended to hit the Cobra.

"I don't think a chopper would have been too hard to hit if they took aim," he said. "But we need to remind them that we take these things seriously. Maybe next time they won't miss."

Sims said the L39 fired two to four rockets at the Cobra without warning. He said he did not know the type and range of the air-to-air missiles, or how close they came to the Cobra. Nor could he say how near the Czechoslovak plane came to the helicopter before launching the attack.

Another L39 was in the area but did not enter West German airspace, according to Sims.

The spokesman said the Cobra, which was armed with 20mm guns, made no attempt to return the fire in what would have been a "mismatch" if a confrontation with the jet fighter had occurred.

Sims said the attack took place about 1 a.m. Saturday in the southeast corner of West Germany, in an area wedged between Czechoslovakia to the north and Austria to the south.

Two separate groups of West German citizens witnessed and confirmed the attacks, according to Sims.

The Cobra, assigned to the 2nd Armored Cavalry at Feucht Army Airfield near Nuremberg, was conducting a routine reconnaissance mission, he said. U.S. aircraft patrol the border every day as a "confidence-building measure" to make sure there are no unusual fortifications, troop movements or changes in troop composition, he said.

A Pentagon official said the Cobra's pilot and copilot did not initially realize they were targets of an attack and thought their helicopter was backfiring.

After observing the smoke of rocket fire, they spotted the L39 and dropped in altitude to evade the attack, according to the official. The crew then landed in a field and checked the craft for damage before returning to their base, he said.

Sims said the helicopter crew was experienced and well trained for surveillance flights. The pilot had 680 hours of flight time, including 250 hours in the border region.