MANAGUA, NICARAGUA, JUNE 20 -- U.S.-backed Nicaraguan rebels shot down a Soviet-supplied helicopter gunship, killing all three crew members, the Defense Ministry said today.

{A spokesman for the Nicaraguan Resistance in Tegucigalpa, Honduras, said there were 18 persons aboard the helicopter and all of them were killed, Reuter reported.}

The Mi25 helicopter was hit by rebel fire yesterday as it supported Sandinista troops in combat in central Nicaragua, about 90 miles northeast of Managua.

It was the first time Nicaragua has announced the loss of an Mi25, the export model of the Mi24 helicopter used extensively by Soviet forces in Afghanistan. Armed with rockets, cannon and machine guns, the helicopter has been used to inflict heavy casualties on the rebels, military experts said.

The helicopter was the third shot down since 1985, according to the government, and the sixth, according to rebels.

The Defense Ministry did not specify what sort of fire had brought down the helicopter. The insurgents are known to possess ground-to-air heat-seeking missiles such as the Redeye. Military experts said the Mi25 is well protected from attack from the ground. Its armor protects it from gunfire and the helicopter is believed to possess a special defense system for diverting heat-seeking missiles.