Japan Airlines said it temporarily grounded one of its Boeing 787 Dreamliners at Tokyo's Narita International Airport on Tuesday after white smoke was spotted outside the plane and a battery cell showed clear signs of leaking.

The incident raised fresh concerns about the 787's safety and reliability almost exactly one year after the global Dreamliner fleet was grounded by regulators following the overheating of two such batteries, although Boeing said design changes made as a result had worked as planned.

Boeing said it was "aware of the 787 issue that occurred Tuesday afternoon at Narita, which appears to have involved the venting of a single battery cell." Venting is the process of fumes and heat being channeled outside the battery casing and the aircraft when the battery overheats.

"The issue occurred during scheduled maintenance activities with no passengers on board," said Boeing. "The improvements made to the 787 battery system last year appear to have worked as designed."

The incident, which was disclosed by Japan Airlines early on Wednesday local time, came nearly a year to the day after Japan Airlines and All Nippon Airways grounded their 787 fleets after two 787 batteries overheated on two different planes in less than two weeks.

Global regulators grounded the worldwide fleet on January 16, 2013. The 787s remained grounded for more than three months while Boeing redesigned the battery, charger and containment system to ensure battery fires would not put the airplane at risk. The cause of the battery problems has not been determined.

United Airlines, the only US carrier that uses the 787, was conducting precautionary checks on its 787s, according to a person familiar with the matter who asked not to be named because they were not authorized to speak publicly.

United declined to comment on the inspections, saying only that "Our 787s are operating normally and we have not experienced any issues with our batteries."

Japan Airlines said maintenance engineers who were in the cockpit saw white smoke outside the plane. When they went outside the aircraft the smoke had dispersed.

On returning to the cockpit, the engineers found warning lights indicating possible faults with the main battery and charger. When they checked the battery, located inside a steel containment box, they found one of eight cells was leaking a liquid.

Japan Airlines said that inside the containment box inspectors found a pressure relief valve had opened in one of the battery's eight cells. The valve in the battery case is designed to open when pressure rises inside a cell, Japan Airlines said.

The problem did not appear to have propagated to other cells in the battery, said a person familiar with the matter, who was not authorized to speak publicly.

The liquid that leaked out also did not appear to breach the containment box, and it appears that any fumes vented through a port that is part of the containment system, a sign the system likely worked properly, this person said.

The plane, due to depart from Narita for Bangkok, was taken out of service, and the 158 passengers due to board the plane were put on a separate 787, JAL said.

PLAGUED WITH PROBLEMS

The 787 Dreamliner is Boeing's state-of-the-art plane, built with carbon-fiber composite materials and a powerful electrical system to reduce weight and improve the jet's fuel efficiency.

But the 250-seat jetliner, which costs about $212 million at list prices, has been plagued with problems. It was more than three years late in entering service, due to issues with parts fabrication by suppliers around the world. Since entering service, it has had issues with brakes, fuel lines, electrical panels and hydraulics, and other systems.

The overheating of the jet's lithium-ion batteries raised serious concerns last year, prompting world-wide grounding of the fleet after a fire on a Japan Airlines plane in Boston and a second battery that overheated on an All Nippon Airways flight in Japan less than two weeks later.

In July, after the 787 was cleared to return to service, an Ethiopian Airlines jet caught fire at London's Heathrow Airport, scorching the fuselage. The cause of the fire was never firmly established, but UK investigators traced the probable cause to faulty wiring of a lithium battery in an emergency beacon located in the ceiling near the tail of the plane. - Reuters