TOKYO (Reuters) - Japan's transport ministry has launched an investigation into what caused two fuel leaks on a Boeing Co 787 Dreamliner jet operated by Japan Airlines Co, just days after U.S. authorities began a wide-ranging review of the aircraft following a series of incidents, including a battery fire.

Over the weekend, the JAL jet, which was undergoing checks in Japan following a fuel leak at Boston airport in the United States last week, again leaked fuel during tests. Both leaks were due to separate valve-related problems.

"Given the fuel leakage incidents occurred in succession, the minister directed the aviation director of the Regional Civil Aviation Bureau to open an investigation and find the causes of the two leaks as soon as possible," the ministry said in an emailed statement on Monday, adding it was in contact with Boeing and the U.S. Federal Aviation Administration.

Boeing said it was aware of the JAL fuel leaks and that it would continue to help the airline and regulators understand the cause and take appropriate action.

Announcing their review on Friday, U.S. transportation officials, and Boeing, said the aircraft was safe to fly, but they needed to take a comprehensive look at the plane to ensure there were no flaws that should be remedied.

Japan is the biggest market so far for the Dreamliner, with JAL and local rival All Nippon Airways Co flying 24 of the 50 Dreamliners delivered to date.

A series of recent incidents - the fuel leaks, the battery fire, a wiring problem, brake computer glitch and cracked cockpit window - have focused intense scrutiny on the new plane.

While many of the mishaps are considered routine for a new design entering service, their occurrence in quick succession on an aircraft boasting major new technology has sparked concerns about its safety.

The 787, the world's first mainly carbon-composite airliner, is Boeing's boldest effort to revolutionize commercial aviation by using new technology to cut fuel costs by around a fifth. Each lightweight jet has a list price of $207 million.

Airlines are pleased with the savings, and have so far given the plane their approval, both by ordering more than 800 of the jets and mostly sticking by it through its current troubles.

(Reporting by James Topham and Alwyn Scott; Editing by Ryan Woo, Ian Geoghegan and Tim Dobbyn)