TOKYO (AFP) -- Japanese engineers probing problems with Boeing's Dreamliners have identified the cause of fuel leaks but are still investigating more serious issues with the aircraft's batteries, the transport ministry said.

Last month, a Japan Airlines Boeing 787 due to fly to Tokyo was grounded in Boston following a fuel spill, while the airline also found a similar problem with another of the next-generation jets in Tokyo.

The transport ministry said Friday that the leaks were caused by defective paintwork and impurities getting into a fuel valve, adding that it had already ordered the airline to take measures to remedy the problem.

"As for the battery issue, our Transport Safety Board and the aviation bureau are continuing to investigate the causes" with their US counterparts, Transport Minister Akihiro Ota said in comments posted on the ministry's website.

All 50 of the 787s in service have been grounded for more than five weeks, disrupting airline schedules, as investigators try to pinpoint the cause of a battery fire on a parked plane and a battery smoke incident that forced another into an emergency landing.

While Boeing has reported progress in the probe, the extent of the problems and the potential complexity in addressing them remains unknown, triggering mounting speculation as to how long the grounding will last.

On Friday, US air safety regulators said Boeing had proposed a fix for battery problems on the Dreamliner but the aircraft would stay grounded worldwide until a safe solution is determined.