The US transportation secretary still has a "lot of questions" about Boeing's grounded 787 Dreamliner, indicating the aviation giant faces a tough battle to get the planes back in the air soon.

"I have made it very clear that I want a thorough review" of the Boeing plan to fix the plane's battery system in order to resume commercial flights, Transportation Secretary Ray LaHood told the Wall Street Journal Wednesday.

"I am going to ask a lot of questions" before a final decision is made, said LaHood, who oversees the Federal Aviation Administration (FAA) and must sign off on any decision.

LaHood announced in January that he would be stepping down once a replacement is selected by President Barack Obama and confirmed by the Senate.

The company is anxious to get its proposals cleared, and on Monday Boeing Commercial Airplanes CEO Ray Conner said it would move "really fast" to get the planes back in the air once the FAA signs off.

After 200,000 hours of analysis and tests, "we feel very good about the fix. We've covered the waterfront, so to speak," he told a conference in New York.

Burnt lithium-ion batteries on two 787s -- a fire on a parked airplane in Boston and smoke that caused an emergency landing in Japan -- resulted in the January 16 global grounding of all 50 787s in service.

The Boeing CEO said the company would stick with the lithium-ion batteries, which are significantly more powerful and lighter than the nickel-cadmium batteries traditionally used on aircraft.

As for the safety implications of the proposed battery fix, Conner said: "We would not go forward unless we thought we had it nailed."

Despite the 787 problems, which have halted deliveries, Conner said that Boeing was keeping its forecast to deliver more than 60 787s this year.