A U.S. Air Force F-15C fighter jet from Kadena Air Base crashed into the sea south of Okinawa, Japan, early Monday morning local time, according to defense officials.

The pilot ejected and was later rescued; nobody else was on the plane, the Air Force said.

The aircraft "was conducting a routine training mission," officials added, saying investigators would look into what may have sparked the crash.

It marked the sixth non-combat crash for the U.S. Air Force in 2018, following seven last year.

The Air Force had ordered a series of safety stand-downs of flying and maintenance wings so that crews can work to determine how to prevent future crashes, the Air Force Times reported.

Last month, an Air Force training jet crashed in Mississippi during a routine training flight. Both pilots ejected safely. That crash unfolded a few hours before Defense Secretary Jim Mattis gave the commencement address at the U.S. Air Force Academy.

Weeks earlier, an Air Force cargo plane crashed outside Savannah, Ga., killing all nine aboard.

Kadena Air Base is located on Okinawa.

Fox News' Mike Arroyo contributed to this report.