WASHINGTON (AFP) - United Airlines said on Tuesday it expected to keep its Boeing 787s out of service through March 30, a fresh sign of how long the aircraft may be kept grounded worldwide to fix a battery problem.

United, the only US airline that operates Boeing's cutting-edge aircraft, said it was scheduling alternate planes through March 30 while its seven 787 Dreamliners remained grounded.

"We are further adjusting our published flight schedules to show alternate aircraft in place of our six Boeing 787s through March 30," Christen David, a spokesman for United, said in an emailed response to an AFP query.

"We will make further adjustments in the future as we gain more visibility on the question of when service will be restored," she said.

All 50 787s in service around the world have been banned from flight since mid-January after battery smoke forced an emergency landing of one and a battery fire occurred on another.