Airline regulators will not be rushed into clearing grounded Boeing 737 Max 9 planes after a mid-air blowout, the US government has said.

The aircraft, which were suspended in the US following an incident on an Alaska Airlines flight “need to be 100% safe”, said Transportation Secretary Pete Buttigieg.

It is unclear when the planes will be allowed to fly again.

Boeing’s boss, Dave Calhoun, described the incident as a “quality escape”.

It means the incident was caused by some failure in quality control in the plane, which had been in service for just eight weeks before the blowout.

Mr Calhoun told CNBC that there were still questions that need to be answered about how the incident was allowed to happen. “What broke down in our gauntlet of inspections? What broke down in the original work that allowed for that escape to happen,” he said.

Earlier this week, Mr Calhoun admitted that Boeing was at fault after a section of the fuselage from a 737 Max 9 plane operated by Alaska Airlines blew-out within minutes of take-off.

No-one was injured when the panel – or door plug – broke away from the Alaska Airlines flight from Portland, Oregon, forcing the plane to make an emergency landing.

The US Federal Aviation Administration (FAA) grounded 171 Boeing jets on Saturday that were installed with the same door plug.

The door plug is a piece of fuselage, with a window, that fills the space where an emergency exit would be in certain configurations.

Mr Buttigieg declined to say when the suspension will end. “The only consideration on the timeline is safety,” he said. “Until it is ready, it is not ready. Nobody can or should be rushed in that process.”

Alaska Airlines has cancelled about 20% of its flights after 65 of its Max 9s were grounded. United Airlines, the other US 737 MAX 9 operator, has 79 of the planes in its fleet out of action.

It said it expects “significant” cancellations on Thursday after 167 flights did not go ahead on Wednesday.

Alaska Airlines said it still needs revised inspection and maintenance instructions from Boeing, which must be approved by the FAA before it can begin flying the planes again.

“We will only return these aircraft to service when all findings have been fully resolved and meet all FAA and Alaska’s stringent standards,” the airline said.

Both Alaska and United said on Monday they had found loose parts on a number of the grounded aircraft.

United said it had found bolts in need of “additional tightening” during inspections of the door plug that came away from the Alaska Airlines plane.

The part which fell off was eventually found in a teacher’s back garden without its four bolts.

National Transportation Safety Board chair Jennifer Homendy said on Monday it’s possible the bolts were missing from the start but they may have come off in the descent.