MANAGUA, NICARAGUA, JUNE 13 -- A Soviet-supplied Mi17 helicopter ferrying government troops and supplies crashed in a northern war zone yesterday, killing all 15 soldiers on board, the Defense Ministry said today.

U.S.-backed Nicaraguan rebels claimed they shot down the helicopter, but the Defense Ministry said that "first reports indicate that the crash was caused by an accident."

The ministry said the Sandinista Air Force was investigating the incident.

The ministry statement said the Soviet-made armored helicopter crashed about 170 miles north of Managua in northern Jinotega province. The aircraft was carrying out support operations with combat troops along the Amaka River in a combat zone, the ministry said.

The river has been a major infiltration and supply route for the U.S.-backed contras seeking to overthrow the Sandinista government.

The contras said in a broadcast over their clandestine Radio Liberation that a rebel-fired surface-to-air missile shot down the helicopter.

The rebels in the past have claimed to have knocked out a number of Nicaraguan helicopters with the use of U.S.-supplied surface-to-air antiaircraft missiles.

On May 10, Nicaragua confirmed that rebels downed a Mi17 helicopter with an antiaircraft missile in the Amaka River region, killing the four soldiers aboard.