An All Nippon Airways (ANA) Boeing 787 Dreamliner is seen after making an emergency landing at Takamatsu airport in western Japan, in this photo taken by Kyodo, January 16, 2013. The plane made the landing in Takamatsu after smoke appeared in the plane's cockpit, but all 137 passengers and crew members were evacuated safely, Osaka Airport said.[Photo/Agencies]

WASHINGTON - US safety regulators are nowhere near finishing an investigation into a battery fire on the Boeing Co 787 Dreamliner, a top official said on Thursday, raising the prospect of a prolonged grounding for the plane.

Airlines have canceled hundreds of flights in the eight days since the plane was grounded, and Boeing has stopped deliveries of newly built jets.

The full financial impact on the planemaker is still not clear. Still, Boeing shares are actually up 1.3 percent since regulators said the plane - full of high-tech innovations that are supposed to be a model for future aviation - could not fly.

Deborah Hersman, chairman of the National Transportation  Safety Board, made clear that investigators have found a series of "symptoms" in the battery damaged in a Jan 7 fire in Boston, but not the underlying cause of the problem.

"We are early in our investigation, we have a lot of activities to undertake," Hersman told a news conference.

"This is an unprecedented event. We are very concerned. We do not expect to see fire events on board aircraft. This is a very serious air safety concern."

She rebuffed multiple questions on how long the investigation would take, making clear it could be weeks or more. She also would not say when the 787 would fly again, which is in the hands of the Federal Aviation Administration.

Former NTSB Chairman Mark Rosenker said the briefing made it clear that the investigators had come up short in their hunt for the cause of the battery fire.

"It's going to take them longer," he said in an interview. "Weeks, not days."

Boeing and its regulators have said they do not know when the 787 will fly again. It has been grounded worldwide since a  plane by All Nippon Airways made an emergency landing in Japan on Jan 16 after a battery incident, which Hersman said may or may not have been a fire.

That emergency landing came after a fire occurred on a Japan Airlines Co Ltd 787 on the tarmac in Boston.

Boeing was not immediately available to comment on the latest NTSB statement. France's Thales, which makes the 787 battery system, declined to comment. 

Other new planes had problems when they were introduced, but not fires, which makes this situation stand out, Rosenker said.

"Fire is something you don't fool with," he said. "You've got to understand that, particularly given the short period of time the aircraft has been flying."

Delays persist

The NTSB and its Japanese equivalent are working together on their probes, though Hersman again insisted the work was still in the early stages.

"It is really very hard to tell at this point how long this investigation will take. We have all hands on deck," she said. "We're working as hard as we can to identify what the failure mode is here and what corrective actions need to be taken."

The 787 program was already years behind schedule before last week's grounding, which means Boeing cannot deliver newly manufactured planes to customers.

That means customers like United Continental Holdings Inc  may have to wait even longer for planes on order. The company's United Airlines already flies six Dreamliners.

"History teaches us that all new aircraft types have issues and the 787 is no different," United Continental Chairman and Chief Executive Jeff Smisek said during the carrier's earnings conference call. "We continue to have confidence in the aircraft and in Boeing's ability to fix the issues, just as they have done on every other new aircraft model they've produced."

Smisek said Thursday the carrier still expects to take delivery of two more 787s in the second half of the year.

Boeing has already delivered 50 of the 787s. Around half have been in operation in Japan, but airlines in India, South America, Poland, Qatar and Ethiopia are also flying the planes, as is US carrier United.

The grounding of the Dreamliner, an advanced carbon-composite plane with a list price of $207 million, has already forced hundreds of flight cancellations worldwide.  

Airbus impacts 

The head of Boeing's European rival Airbus said it would study the 787 Dreamliner design review and make any changes to its future A350 jetliner that may be needed as a result of the US findings.

"We believe so far we have a robust design, however we will draw the lessons from the 787," Airbus Chief Executive Fabrice Bregier told Reuters Television at the World Economic Forum in Davos.  

"We will look at the recommendations and guidelines of the FAA and if by chance we need to change it we have plenty of time because this aircraft, the 350, will be delivered to our first customers not before the second half of 2014  so it is not a challenge and it is not a burden for us."

Billed as Europe's response to the Dreamliner, the A350 is due to enter service next year using lithium-ion batteries but without the same reliance on electrical systems as the 787, something Airbus says will put less burden on the batteries.

However, Airbus has so far declined to comment on how it would tackle a battery fire if one did break out on board.

One industry veteran said airline customers need absolute certainty that Boeing and regulators have solved the problem.

"You don't need details here to understand why people are terrified about the possibility of these batteries catching fire," Virginia-based consultant Loren Thompson said.