Nearly a month after the high-tech F-35A plummeted into the sea off the coast of northeastern Japan, neither the pilot's body, nor the plane's fuselage have been found, said Defence Minister Takeshi Iwaya.

Experts say Japan and the US are keen to prevent debris from the plane being recovered by Russia or China, with Iwaya admitting last month there were "a significant amount of secrets that need to be protected" on board.

Some debris, including a part of the flight data recorder, were recovered "on or after May 3", Iwaya said, adding: "The Defence Ministry is studying (the parts), but at this point, the all-important memory (of the flight data recorder) has not been recovered."

The fighter jet went missing on April 9 while flying 135 kilometres (85 miles) east of Misawa, northeastern Japan, on a training mission.

The plane lost contact about 30 minutes after taking off from Misawa Air Base with three other aircraft.

It was the first reported case of a crash by an F35-A, according to Japan's Air Self-Defence Force.

Japanese and US search crews have already found the jet's tail.

US and Japanese troops have used data from a Japanese seabed research ship along with a US-chartered special "diving support" vessel for deep-sea operations to pull up the newly discovered parts, Iwaya said.

Japan is deploying F35-As, each of which costs more than 10 billion yen ($90 million), to replace its ageing F-4 fighters.

They are a key part of Prime Minister Shinzo Abe's efforts to upgrade the nation's military capacity to meet changing power dynamics in East Asia, with China rapidly modernising its military.