The US air safety regulator announced Saturday it was grounding some Boeing 737 MAX 9 airplanes pending inspections, a day after a panel blew out of one of the planes over the western state of Oregon. 

The Federal Aviation Administration "is requiring immediate inspections of certain Boeing 737 MAX 9 planes before they can return to flight," the agency said on X, formerly Twitter. 

The agency said around 171 aircraft worldwide would be affected, with each inspection taking four to eight hours. 

"Safety will continue to drive our decision-making," the FAA statement said.

Alaska and United Airlines fly the largest number of MAX 9 planes, while Icelandair and Turkish Airlines have smaller fleets of the aircraft.

Boeing has so far delivered about 218 737 MAX 9 planes worldwide, the company told AFP.

US-based Alaska Airlines grounded all 65 of its Boeing 737 MAX 9 planes on Friday after a flight carrying 171 passengers and six crew was forced to make an emergency landing, with the National Transportation Safety Board (NTSB) saying a sealed-over door panel had opened and come off mid-flight.

Alaska Flight 1282 had departed from Portland International Airport and was still gaining altitude when the cabin crew reported a "pressurization issue," according to the FAA. 

The plane quickly returned to Portland, and there were no major injuries.

Images posted on social media showed a side panel of the plane blown out, with emergency oxygen masks hanging from the ceiling.

"Following tonight's event on Flight 1282, we have decided to take the precautionary step of temporarily grounding our fleet of 65 Boeing 737-9 aircraft," Alaska Airlines CEO Ben Minicucci said in a statement Friday.

"Each aircraft will be returned to service only after completion of full maintenance and safety inspections," he said.

Passenger Kyle Rinker told CNN the problem occurred soon after takeoff.

"It was really abrupt. Just got to altitude, and the window/wall just popped off," he told the broadcaster.

The NTSB said no one was sitting in the two places nearest the panel, but the Oregonian newspaper quoted passengers as saying a young boy seated in the row had his shirt ripped off by the sudden decompression, injuring him slightly. 

Another passenger, Vi Nguyen, told The New York Times that a loud noise during the flight had woken her. 

"I open up my eyes and the first thing I see is the oxygen mask right in front of me," Nguyen told the newspaper. "And I look to the left and the wall on the side of the plane is gone."

"The first thing I thought was, 'I'm going to die,'" she added.

Aviation specialist John Ostrower, of the Air Current website, said the affected panel was a "mid-aft door," which, for some carriers, Boeing deactivates before delivering the new planes. 

To passengers, the panel would appear like a normal window, he said.

The NTSB dispatched a team to Portland to examine the craft. The board's chair, Jennifer Homendy, said it was "very, very fortunate" that the incident had not ended in tragedy.

"We have the safest aviation system in the world. It is incredibly safe," she said. "But we have to maintain that standard."

Homendy revealed that the door had fallen off over the Portland suburb of Cedar Hills, and called on residents to come forward if they found it.

FAA and Alaska Airlines said they were also investigating.

"While this type of occurrence is rare," the airline said in an earlier statement, "our flight crew was trained and prepared to safely manage the situation."

Alaska Airlines said Saturday that more than a fourth of its Max 9 fleet had been inspected since the incident, with nothing noteworthy being found.

The plane, which had been headed to Ontario, California, was certified airworthy in October and was newly delivered to Alaska Airlines, according to the FAA registry website.

"Safety is our top priority and we deeply regret the impact this event has had on our customers and their passengers," Boeing said in a statement. 

"We agree with and fully support the FAA's decision to require immediate inspections of 737-9 airplanes with the same configuration as the affected airplane."

United, which has the world's largest fleet of 737 MAX 9s, said it grounded 46 of these planes and that 33 have now been inspected. This was expected to cause 60 flight cancellations on Saturday.

Aeromexico said it was grounding all of its 737 MAX 9 planes while inspections are carried out.  

Icelandair said none of its 737 MAX 9's featured the plane configuration specified in the FAA grounding order.

The Panamanian carrier Copa Airlines said it was grounding 21 of its 737 MAX 9s in compliance with the FAA order and will inspect them.  

Boeing has struggled in recent years with technical and quality control issues related to its 737 MAX models. 

In December, the US aviation giant told airlines that MAX aircraft should be inspected to check for loose hardware on plane rudder control systems after an international operator discovered a bolt with a missing nut while performing routine maintenance.

Boeing's 737 MAX planes were grounded worldwide following two MAX 8 crashes in 2018 and 2019 that killed 346 people in total.

The FAA approved the planes' return to service only after the company made changes to its flight control system.