An American F-15 fighter crashed into the sea after its pilot ejected from his plane off Japan's southernmost Okinawa prefecture on Tuesday, the U.S. Air Force said.

"A Kadena-based F-15 aircraft developed a problem that ultimately resulted in the pilot ejecting from the aircraft over the Pacific Ocean approximately 70 miles (110 kilometers) east of Okinawa," the air force said, referring to the hub of American air power in the Pacific.

The tropical island chain of Okinawa Tuesday marked 40 years since U.S. occupying forces returned it to Japan, as locals readied to protest against the continued American military presence there.

A ceremony, attended by Prime Minister Yoshihiko Noda, U.S. ambassador John Roos and a phalanx of local politicians, is due to be held later in the day marking the anniversary.

A strong 6.8 magnitude earthquake struck in waters off Okinawa island on Tuesday, but there were no concerns about a tsunami, the U.S. and Japanese agencies said.

The quake occurred in the East China Sea, 218 kilometers (136 miles) west of Naha, Okinawa prefecture, the U.S. Geological Survey said. The quake's depth was 222 kilometers (138 miles).