WASHINGTON — The US aviation regulator says it will allow Boeing's 737 Max 9 jets to resume flying after inspections are completed.

The Federal Aviation Administration (FAA) grounded 171 of the planes after an unused door broke away mid-flight.

United Airlines and Alaska Airlines plan to start returning the jets to service in the coming days.

But the FAA says it will not yet allow Boeing to expand production of its best-selling narrow body family of jets, which includes the 737 Max 9.

The unprecedented decision by the regulator marks more bad news for the plane making giant.

"This won't be back to business as usual for Boeing," FAA Administrator Mike Whitaker said in a statement.

"We will not agree to any request from Boeing for an expansion in production or approve additional production lines for the 737 Max until we are satisfied that the quality control issues uncovered during this process are resolved," he added.

The announcement that the planes will be allowed to fly again once inspections have been completed is welcome news for Alaska Airlines and United Airlines, which had to cancel thousands of flights.

Alaska Airlines said it expects "to bring our first few planes back into scheduled commercial service on Friday", with more planes added every day as inspections are completed.

United Airlines said it had received final approval from the FAA to complete the process of returning its 79-strong fleet of 737 Max 9 planes to service.

The company's chief operations officer Toby Enqvist said the airline was preparing to start flying the aircraft again from 28 January.

On 5 January a door plug on an Alaska Airlines 737 Max 9 blew off shortly after take-off, terrifying passengers, and forcing an emergency return to the Portland, Oregon airport.

Bosses of both Alaska Airlines and United Airlines have expressed frustration with Boeing over the grounding of the 737 Max 9, which has caused major disruptions to their services.

In an interview with NBC News, Alaska Airlines boss Ben Minicucci said there was "no doubt" that the plane came "off the production line with a faulty door".

He said airline inspections since the incident found "many" loose bolts. "I'm more than frustrated and disappointed," he said. "I am angry."

United Airlines chief executive, Scott Kirby, also told CNBC that he is "disappointed".

"The Max 9 grounding is probably the straw that broke the camel's back for us," he said, adding that "we're going to build a plan that doesn't have the [Boeing] Max 10 in it".

United also said earlier this week that it expected to lose money because of the grounding.

On Wednesday, the head of Boeing, David Calhoun, faced questions from lawmakers in Washington to explain what led to the mid-air emergency.

The latest incident raises fresh questions about the safety of Boeing's aircraft.

The company faced intense scrutiny after two fatal crashes of 737 Max 8 passenger jets in 2018 and 2019, which killed 346 people. — BBC