US air-safety regulators said Friday they were reviewing a Boeing proposal to fix battery problems on the 787 Dreamliner that have kept the aircraft grounded worldwide since mid-January.
After meeting with senior Boeing executives to discuss the ongoing investigation of the battery issues, the Federal Aviation Administration said that Boeing had offered a plan to address the problems.
"The FAA is reviewing a Boeing proposal and will analyze it closely," the regulator said in a statement.
"The safety of the flying public is our top priority and we won't allow the 787 to return to commercial service until we're confident that any proposed solution has addressed the battery failure risks."
Boeing said minutes earlier that it was "encouraged" by progress in the air-safety investigation.
Ray Conner, the head of Boeing Commercial Airplanes, had a "productive meeting" with FAA chief Michael Huerta in Washington, a Boeing spokesman said in a statement.
"We are encouraged by the progress being made toward resolving the issue and returning the 787 to flight for our customers and their passengers around the world," he said.
All 50 of the 787s in service around the world have been banned from flight since January 16 after a battery fire occurred on a parked plane and battery smoke on another one forced an emergency landing.
US and foreign investigators have reported progress in the probe of the lithium-ion batteries but have yet to pinpoint the cause of the problems.
Boeing said it was working around the clock with teams of hundreds of experts on the issue and working closely with the FAA and other authorities.
"We greatly appreciate all the resources that have been dedicated to this effort across the FAA by administrator Huerta," the spokesman said.
The New York Times has reported, citing industry and federal officials, that Boeing has narrowed down the ways in which the lithium-ion batteries could fail and concluded they would be safe to use after making changes, such as adding insulation between the battery cells.
Boeing has declined to comment on the reports.
Boeing halted 787 deliveries shortly after the planes were grounded but continues to produce 787s at a rate of five per month.
Airlines flying the cutting-edge, fuel-efficient plane have been forced to cancel flights and rearrange schedules due to the groundings, while postponed deliveries have put strains on waiting customers.
On Thursday, United Airlines said it was taking its six Boeing 787s out of service through June 5, except to launch a Denver-Tokyo route on May 12 if circumstances allowed.