TOKYO -- The U.S. military says an Air Force F-15C fighter jet crashed in waters off Japan's southern island of Okinawa Monday. The Air Force said in a statement that the pilot ejected and had been rescued.

A subsequent tweet offered an update:

We have been notified that the pilot is in serious condition. Our hearts and prayers go to the pilot and his family as they go through this difficult time. @USAirForce @PacificCommand @USForcesJapan @PACAF @KadenaAirBase_J — Kadena Air Base (@KadenaAirBase) June 11, 2018

The cause of the 6:30 a.m. crash is under investigation. The F-15 from the Air Force's Kadena Air Base in Okinawa was on a routine training mission.

Get Breaking News Delivered to Your Inbox

The U.S. has a heavy military presence in Okinawa that has been the source of friction with the local government and some residents. About half of the 50,000 American troops in Japan are stationed on the southern island.

Many Okinawa residents blame the U.S. bases for crime, accidents and pollution and want them moved off the island, the Reuters news agency points out, adding that, "A series of emergency landings and parts falling from U.S. military aircraft have highlighted safety concerns."