US authorities are stepping up oversight of Boeing's manufacturing line a week after a door plug broke off one of its planes mid-flight.

The head of the Federal Aviation Administration (FAA) said he believed there were "significant problems" with the 737-9 Max jet as well as "other manufacturing problems".

The FAA said it would conduct an audit of the plane's production line.

It also plans to review who is in charge of quality oversight.

For years, the FAA has delegated some parts of quality reviewing of planes to Boeing, but the practice has been controversial, drawing repeated warnings of safety risks.

"It is time to re-examine the delegation of authority and assess any associated safety risks," FAA administrator Mike Whitaker said in a statement.

"The grounding of the 737-9 and the multiple production-related issues identified in recent years require us to look at every option to reduce risk. The FAA is exploring the use of an independent third party to oversee Boeing's inspections and its quality system."

Boeing did not immediately comment.

The US aerospace giant has been struggling to restore confidence after crashes in 2018 and 2019 involving a different plane in the 737 Max group killed 346 people.

Poor design of a piece of its flight control system was found to play a role, and authorities grounded its popular 737 Max planes globally for more than 18 months. Lax oversight by the FAA was also faulted.

Boeing has reported a string of smaller manufacturing issues as production resumed.

The blowout on the Alaska Airlines flight from Portland, Oregon to California, which forced an emergency landing but resulted in no serious injuries, has revived scrutiny.

Passengers on the Alaska Airlines flight have filed a class action lawsuit against Boeing. Their lawyers described the incident as a "nightmare experience" that had caused "economic, physical and ongoing emotional consequences that have understandably deeply affected our clients".

Mark Pegram, the father of Sam Pegram, an aid worker who died in the Ethiopian Airlines 2019 crash, said the emergency should make the government reconsider the deal it made with Boeing after the the earlier fatal accidents, in which the firm paid $2.5bn but may see its criminal charge dropped.

He also called for an independent industry monitor.

"The need for this independent oversight is even more necessary now when one considers the ineptness of the FAA which has failed again to properly oversee Boeing's Max aircraft production, as evidenced by these latest near catastrophes," he said.

"The lives of passengers should not needlessly, and once again, be put at risk."

The National Transportation Safety Board (NTSB) is investigating what happened on the Alaska Airlines plane, which had been in service for just eight weeks.

On Thursday, the FAA formally announced an inquiry into Boeing's processes.

It has also grounded 171 planes - most of the 737-9 Max fleet - with similar door plugs for inspections, which have revealed loose bolts and other issues.

Boeing has said it will cooperate fully and transparently with the probes. Supplier Spirit Aerosystems is also involved.

Boeing chief executive Dave Calhoun has already said the company will admit its "mistake" and has described the issue as a "quality escape".

The FAA's Mr Whitaker told CNBC that officials believed there were "other manufacturing problems".