(CNN) —  

A U.S. military helicopter crashed at a Marine base in Okinawa on Monday, the Japanese defense ministry said.

Three of the four crew members on board were safe, Defense Ministry spokesperson Atsushi Sakurai said. The condition of the fourth crew member and the cause of the crash were not immediately clear.

The crash happened at Camp Hansen, a U.S. base on the southern Japanese island of Okinawa. The camp is named after Dale M. Hansen, a Marine who was honored for his part in the fight for Hill 60 during the Battle of Okinawa.