WASHINGTON (REUTERS) - A top Boeing Co executive said on Tuesday it was too early to determine whether problems with the lithium ion batteries on its new 787 Dreamliner that led to the plane's grounding in January stemmed from a design error or some manufacturing problem.

Mr Mike Sinnette, Boeing's chief 787 engineer, told an investigative hearing of the National Transportation Safety Board that the batteries had been through thousands of hours of testing before the new airliner began flying, plus additional hours during flight testing.

He said the batteries were part of a layered system that sought to avert a safety problem on board the airplane. He said there was no evidence that the batteries involved in two issues in Japan had been subjected to any abuse.