The U.S. Air Force released a statement saying that the HH-60 rescue helicopter was on an unspecified training mission when it crashed at Camp Hansen. The statement said the cause of the crash was not known and did not elaborate on the condition of the crew members.
However, Defense Minister Itsunori Onodera said he received information that three crew members ejected from the helicopter and the fourth was apparently injured and taken to a hospital.
Authorities said there were no reports of injuries or damage outside the base.
The rescue chopper belongs to Okinawa's Kadena Air Base.
The crash comes amid strong local opposition to the U.S. Marine Corps' additional deployment of 12 MV-22 Osprey transport aircraft on the island. About half of the 50,000 U.S. troops in Japan are based on Okinawa under a Japan-U.S. security pact.
In 2004, a CH-53 helicopter from Marine Corps Air Station Futenma crashed into a nearby university building. The accident triggered a large anti-base uproar. The crew survived, and there were no civilian injuries.
The Associated Press contributed to this report.