TOKYO, June 11 (Xinhua) -- A U.S. F-15 fighter jet crashed into the sea off Okinawa, Japan's southernmost prefecture, on Monday, with a Maritime Self-Defense Force (MSDF) helicopter rescuing the pilot, according to government sources here.

The crash of the F-15 jet, which belonged to the U.S. Kadena Air Base, occurred at around 6:40 a.m local time and around 80 km south of Okinawa's prefectural capital of Naha, the sources said.

According to sources close to the matter, the condition of the pilot who was rescued by the Japanese side after ejecting from the jet remains unknown.

Okinawa hosts the bulk of U.S. military bases and assets in Japan, yet the island accounts for just a fraction of Japan's total land mass.

There have been mounting calls from officials and locals on the island to have their base hosting burdens reduced with the latest crash of a fighter jet doing little to help ease concerns about the safety of U.S. military assets in Okinawa.

In May 2013, an F-15 fighter crashed into the sea off Okinawa.