* F-35 is first of Japan's stealth fighters
* Military searches for missing stealth jet (Adds comment from Pentagon and Lockheed Martin)
TOKYO, April 9 (Reuters) - Japan's military said on Tuesday it lost contact with one of its Lockheed Martin F-35 stealth fighters over the Pacific Ocean close to northern Japan.
Japan's first squadron of F-35s has just become operational at the Misawa air base and the government plans to buy 87 of the stealth fighters to modernize its air defences as China's military power grows.
The advanced single-seat jet was flying about 135 km (84 miles) east of the air base in Aomori Prefecture at about 7:27 p.m. (1027 GMT) on Tuesday, when it disappeared from radar, the Air Self Defense Force said.
The military has launched a search for the missing aircraft and its pilot, it said in a statement.
Lockheed Martin said in a statement that it was standing by to support the Japanese Air Self Defense Force as needed.
The Pentagon said it was monitoring the situation.
A crash would be only the second time an F-35 has gone down since the plane began flying almost two decades ago. It would also be the first crash of an A version of the fifth-generation fighter designed to penetrate enemy defences by evading radar detection.
A U.S. military short take off and landing (STOVL) F-35B crashed near the Marine Corps Air Station Beaufort in South Carolina in September prompting a temporary grounding of the aircraft. Lockheed Martin also makes a C version of the fighter designed to operate off carriers.
Japan's new F35s will include 18 short take off and vertical landing (STOVL) B variants that planners want to deploy on its islands along the edge of the East China Sea. (Reporting by Tim Kelly, Kiyoshi Takenaka and Stanley White. Additional reporting by Idrees Ali and Chris Sanders in Washington; Editing by Darren Schuettler and Cynthia Osterman)