U.S.-backed contras shot down a Soviet-made MI-17 helicopter providing ground support to Sandinista troops in Nicaragua, the Defense Ministry in Managua reported. There was no word on the fate of its four-man crew. The ministry said the helicopter was shot down Sunday by a U.S.-made Redeye surface-to-air missile near a Sandinista military training base 188 miles north of Managua. The contras' clandestine radio station reported that the rebels shot down a second Sandinista helicopter--an MI-25 military transport--with a Redeye missile Sunday. A Defense Ministry spokesman denied that report.