One week after an F-35A stealth fighter jet crashed off the northeastern coast of Japan, U.S. and Japanese military vessels are struggling to find the wreckage and protect its valuable "secrets."

The Japanese jet vanished from the radar on April 9 over the Pacific as it was conducting a training mission with three other aircraft some 135 kilometers east of Misawa, Aomori Prefecture.

A defense ministry spokesman told AFP that the remains of the jet's tail had been found but they were still hunting in vain for the rest of the fuselage, as well as the pilot.

"On average two aircraft, including a helicopter, and two patrol vessels are constantly deployed in the around-the-clock search operations," said the official, who asked to remain anonymous.

Japan's Maritime Self-Defense Force has also dispatched an unmanned submersible vessel.

Separately, the U.S. military has dispatched one military aircraft and one vessel to join the mission, said the official, adding that the search has not yet been scaled back.