(Adds Japanese defence minister's comment, detail)
TOKYO, June 10 (Reuters) - Japan's air force said on Monday "spatial disorientation" likely caused one of its pilots to fly his F-35 stealth fighters into the Pacific Ocean in April, hitting the water at more than 1,100 kph (683 mph).
The Lockheed Martin Corp jet disappeared from radar screens during an exercise with three other F-35s over the ocean off northwest Japan on April 9. The 41-year-old pilot was killed.
"We believe it highly likely the pilot was suffering from vertigo or spatial disorientation and wasn't aware of his condition," Defence Minister Takeshi Iwaya told a briefing.
"It can affect any pilot regardless of their experience."
By dismissing mechanical or software problems as a cause for the advanced fighter's crash, Japan's assessment is likely to come as a relief to other countries that operate or plan to introduce the jet, including the United States, Britain and Australia.
Japan, Iwaya said, would increase vertigo training for its pilots, check its remaining F-35s and meet residents living near the base from which they operate, in Misawa, Aomori prefecture, before restarting flights.
The air force has yet to recover any intact data from the $126 million aircraft's flight data recorder to back its assessment, which is based on data and communication received by ground controllers and interviews with other pilots.
The pilot, who had only 60 hours flying time in the F-35, gave no indication he was in trouble and did not try to avoid a collision despite advanced instrumentation and a ground proximity warning system that should have alerted him to pull up.
The air force did not find any indication he had attempted to eject.
His plane, which was less than a year old and part of a squadron that had just became operational, crashed 28 minutes after taking off. Debris from the plane is spread across the sea floor about 1,500 metres below where it slammed into the water.
Japan in December said it would buy 45 more F-35 stealth fighters, including some short take-off and vertical landing (STOVL)B variants, worth about $4 billion, adding to the 42 jets it has ordered.
Japan says it eventually wants to field a force of about 150 F-35s, making it the biggest overseas purchaser of the fighter, as it tries to keep ahead of Chinas advances in military technology. (Reporting by Tim Kelly Editing by Robert Birsel)