The U.S. Federal Aviation Administration on Tuesday approved Boeing Co's certification plan for a redesigned battery system on the 787 Dreamliner and outlined a series of tests that must be passed before the 787 can return to service.

The agency said it has approved test flights for two 787 aircraft with the proposed battery system improvements.

"We won't allow the plane to return to service unless we're satisfied that the new design ensures the safety of the aircraft and its passengers," U.S. Transportation Secretary Ray LaHood said in a statement.

The planes were grounded after a fire was reported on a parked plane in Boston in January.

(Reporting by Ros Krasny; Editing by Gary Hill)