NEW YORK — The US airline regulator has ordered the grounding of some Boeing 737 Max 9 jets after an outer section of one of the planes fell off during an Alaska Airlines flight.

The Federal Aviation Administration (FAA) said the inspections will affect 171 planes. On Friday the Alaska Airlines flight had to make an emergency landing in the US state of Oregon.

The plane, carrying 177 passengers and crew, landed safely in Portland.

In a statement, the FAA said it "will order the temporary grounding of certain Boeing 737 Max 9 aircraft operated by US airlines or in US territory".

Required inspections will take around four to eight hours per aircraft, it said.

The Alaska Airlines flight to Ontario, California, had reached 16,000ft (4,876m) when it began its emergency descent, according to flight tracking data.

Images sent to news outlets showed the night sky visible through the gap in the fuselage, with insulation material and other debris also seen.

There were no immediate indications of the cause of the apparent structural failure, nor any reports of injuries.

Evan Smith, one of the 171 passengers on board, said: "There was a really loud bang towards the left rear of the plane and a woosh noise — and all the air masks dropped.

"They said there was a kid in that row who had his shirt sucked off him and out of the plane and his mother was holding onto him to make sure he didn't go with it."

In an audio clip, the pilot can be heard talking to air traffic control requesting a diversion.

"We are an emergency," she said. "We are depressurized, we do need to return back."

According to photographs, the affected area was in the back third of the plane, behind the wing and engines.

The section of fuselage involved appears to be an area that can be used as an additional emergency exit door by some operators, but not by Alaska.

Announcing the initial grounding of 65 planes, Alaska Airlines' CEO Ben Minicucci said: "Each aircraft will be returned to service only after completion of full maintenance and safety inspections."

A later statement said that more than a quarter of those planes had been inspected and would return to service as there were no issues found.

It is the latest problem involving Boeing's best-selling model, which was grounded for almost two years following crashes in 2018 and 2019. — BBC