The Russian Defense Ministry says one of its helicopters has crashed in northwestern Russia, killing its entire crew of four.

The ministry said the Mi-8 helicopter went down Monday during a training mission near Ostrov in the Pskov region, about 600 kilometers (about 370 miles) northwest of Moscow.

The reason for the crash wasn't immediately known. A team of military experts headed to the area to investigate the crash.

The Mi-8, designed in the 1960s, has been the workhorse for both Soviet and Russian aviation and widely exported around the world. Thousands have been built.