WASHINGTON: United Airlines said Thursday it expected to keep its six Boeing 787s out of service until at least May 12, as the cutting-edge airplane remained grounded worldwide due to battery problems.

United, the only US airline with the 787 Dreamliner, just two days ago announced it would keep them on the ground through March 30.

United has "tentatively" scheduled a 787 on its Denver route to Tokyo's Narita International Airport on May 12, UAL spokeswoman Christen David said in an emailed response to an AFP query.

"We are taking the 787 out of our schedule through June 5, except for Denver-Narita," she said.

A person familiar with the case said the Federal Aviation Administration (FAA) investigation of overheated lithium-ion batteries on the 787 was ongoing and schedule adjustments were necessary.

All 50 787s in service around the world have been banned from flight since January 16 after a battery fire 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 is set to propose temporary fixes to the battery problems to US air-safety regulators Friday and could have them back in the air in two months, The New York Times reported.

The Times, citing industry and federal officials, said Boeing had narrowed down the ways in which the lithium-ion batteries could fail, concluding they would be safe to use after making changes such as adding insulation between the battery cells.

Boeing commercial airplane division chief Raymond Conner will unveil the proposals in a meeting Friday with FAA chief Michael Huerta, according to the Times.

Federal officials told the newspaper the aircraft could be back in the air by April if the fixes are approved.

A Boeing spokesman said the aerospace giant was aware of United's 787 schedule adjustments and reiterated the company's regret about the impact of the groundings on its customers.

Asked about the media reports that Boeing intends to submit a proposed battery fix to the FAA Friday, the spokesman declined to comment.

In an interview with the Wall Street Journal published late Thursday, Boeing Chief Executive Jim McNerney explained that he has been working behind the scenes to resolve the crisis.

"I'm the one who has to stand up with absolute confidence when Boeing proposes a solution to enable this technology for the world," he told the newspaper. "And the only way I know how is to dive in deeply with the people doing the scientific and technical work."

He added: "This airplane is our near- and medium-term future, and ultimately speaks to our reputation and our brand."