PANAMA CITY, PANAMA -- U.S.-backed Nicaraguan rebels have significantly hampered government combat operations by downing Sandinista Army helicopters during their current offensive, U.S. military officials here and rebels said.

The rebels and the Nicaraguan Defense Ministry have made differing claims about how many helicopters have been hit, but both sides agreed that the Soviet-supplied choppers continue to give the leftist Sandinista government a crucial edge in mobility in the five-year-old war.

In the most recent incident, the Nicaraguan Defense Ministry acknowledged that the rebels, known as counterrevolutionaries or contras, shot down a Soviet-made Mi25 Hind assault helicopter June 19 near the northern village of Cano Negro, 110 miles northeast of Managua.

The government said three crewmen were killed, while the contras said the dead numbered more than a dozen. A contra communique released in Miami said the chopper was hit with a U.S.-supplied Redeye heat-seeking missile and dropped "under the noses" of a contra guerrilla unit.

The contras said they have destroyed or damaged 13 Sandinista helicopters during an offensive that began early this year: four Mi24 or Mi25 Hind gunships, and nine Mi17 Hip troop transports.

The Defense Ministry said it lost only three helicopters, an Mi24 and two Mi17s, in May and June. The Sandinista fleet has about 50 assault helicopters, including 10 to 12 Hinds, which the Soviet Union provided to Nicaragua virtually free.

The Sandinista Army appears to be preparing to take delivery in coming weeks of another shipment of Soviet helicopters, a U.S. military officer here said, at least the fourth since 1984. Government troops are setting up security measures along the route from the Atlantic Coast port of El Bluff, where the aircraft are unloaded, to Managua.

The helicopters are usually assembled in a Sandinista Air Force hangar at Sandino international airport in the capital, the officer said. The new helicopters were ordered to replace Sandinista losses.

Contra commanders said the chopper hits show that their guerrillas made important advances as a result of U.S. training last year in Florida and Honduras.

"Back in 1985 the Mi24 was a terrifying weapon to our forces. We ran like rabbits when a hawk is overhead," said the contra field commander whose nom de guerre is Mike Lima, interviewed in Miami. "Now our men can defend themselves better. When a helicopter comes, no one moves."

Nicaragua received its first shipment of Mi24 Hinds from the Soviet Union in November 1984. At first U.S. intelligence mistook the contents of the shipping crates for MiG21 fighter jets. Then the Reagan administration mounted an intense but unsuccessful diplomatic effort to stop the helicopters' deployment.

"The Mi24 changed the balance in the Sandinistas' favor. It gave them firepower and mobility," said contra military spokesman Bosco Matamoros interviewed by telephone in Washington. He added: "Now we are in the process of reestablishing the strategic balance. We want to force them to come down from their helicopters and trucks to fight us on the ground where we are more equal."

Lima said contra guerrillas now have limited the movements of Sandinista choppers. But there is no area where the contras can completely prevent the aircraft from flying, he said.

Contra spokeswoman Marta Sacasa, interviewed in Miami, said CIA-trained contra marksmen have twice hit choppers with Redeye missiles. Contras said they hit an Mi17 May 10 with a Redeye in the hamlet of Cuartelon in northern Jinotega province. Based on documents recovered from the wreckage, the contras said the pilot was a Cuban, Jorge Crisalto Odrian, wearing a Sandinista Air Force lieutenant's uniform.

The Nicaraguan Defense Ministry acknowledged that four crewmen were killed in that action, but did not give their nationalities.

It also reported that an Mi17 went down June 12 near San Jose de Bocay in Jinotega, killing all 15 persons on board. The Sandinistas said it suffered mechanical failure. It "probably was an accident," a U.S. military officer here said.

The Redeye missiles, purchased with the final, February installment of $100 million in U.S. aid, are now reaching units in southeastern Nicaragua that are about 40 days' walk from Honduran base camps.

The Mi17 Hip, although it is mainly a light troop transport, is no less effective than the more sophisticated, armored Mi24, the U.S. military officer noted. It can quickly move more than 30 soldiers into combat over Nicaragua's steep hills, resupply them and evacuate wounded. The Hip can be heavily armed with machine and Gatling guns.

The U.S. military officer, explaining the gap between contra and Sandinista claims, said that in many cases contras shoot at government helicopters with automatic rifles and grenades that do damage that is later repaired. A contra gunner on the ground, seeing the aircraft flying away smoking and wobbling, assumes it will crash and reports it as destroyed.

Military strategists in the region have noted the contras could inflict more permanent damage by sabotaging helicopters sitting in bases on the ground.

But Lima flatly rejected that possibility, saying the Sandinistas rarely store their helicopters in the same place for long and always post guards around them. The government reportedly has trained a special unit for helicopter security.