The instructions, known formally as a multi-operator message, are a key step to allow airlines to complete or certify inspections have been conducted that comply with the Federal Aviation Administration's directive that would allow them to put planes back in service.

The message was delivered to airlines around 3 a.m. Pacific Time (1100 GMT), a source told Reuters, who was first to report that Boeing had sent the instructions.

The U.S. Federal Aviation Administration (FAA) did not immediately comment on whether it signed off on Boeing's instructions to airlines or when the airlines could begin to return planes to service. The FAA had said on Saturday it expected the inspections would take four to eight hours.

Alaska Airlines did not immediately comment.

The FAA on Saturday ordered the temporary grounding of 171 Boeing MAX 9 jets after a door plug tore off from a brand-new MAX 9 shortly after it took off from Portland, Oregon.

On Sunday the U.S. regulator said the affected fleet of Boeing MAX 9 planes, including those operated by other carriers such as United, would remain grounded until the regulator was satisfied they were safe.

Boeing shares were down 6.4% in Monday trading, though off the day's worst losses.

(Reporting by Valerie Insinna and David Shepardson; Editing by Chizu Nomiyama)

By Valerie Insinna