(AFP) - Airlines and safety bodies around the world grounded some versions of the Boeing 737 MAX 9 jets pending inspections Sunday, with dozens of flights cancelled over the weekend after a panel blew out of one of the planes over the western US 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.

It added that around 171 aircraft worldwide would be affected, with each inspection taking four to eight hours.

Alaska and United Airlines fly the largest number of MAX 9 planes, while Turkish Airlines has a smaller fleet.

As of Sunday, all three airlines had grounded their jets for inspection.

Boeing has so far delivered about 218 of the 737 MAX 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.

The National Transportation Safety Board (NTSB) said 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 on 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.

- 'Very, very fortunate' -

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

According to Aviation Week magazine, airlines that do not choose MAX models offering the maximum seating configurations can have the door sealed up, which makes it look like a typical window.

The NTSB dispatched a team to Portland to examine the Alaska Airlines 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 urged residents to come forward if they found it.

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

The plane, which had been heading 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.

"A Boeing technical team is supporting the NTSB's investigation into last night's event," it added.

- MAX struggles -

United, which has the world's largest fleet of 737 MAX 9s, said it grounded 46 of the planes and 33 had now been inspected. There were an estimated 60 flight cancellations on Saturday.

Aeromexico said it was grounding all of its 737 MAX 9 planes while inspections were carried out, and Turkish Airlines announced on Sunday that it would also suspend flights of five MAX 9 planes for checks.

Panamanian carrier Copa Airlines said it was grounding 21 of its 737 MAX 9s, while Icelandair said none of its 737 MAX 9's featured the plane configuration specified in the FAA grounding order.

The European Union's Aviation Safety Agency (EASA) said in a statement Sunday it would follow the FAA's directive, but that it doesn't believe that any EU airlines currently operate the 737 MAX 9.

Singapore Airlines, one of Asia's largest carriers, said in a statement it was also not affected because it does not fly the model.

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.

© Agence France-Presse