WASHINGTON (AFP) – The US Federal Aviation Administration said it will order inspections of some 175 older Boeing 737s worldwide after a fuselage rupture forced a US regional flight into an emergency landing.

The announcement came as Southwest Airlines said it had found cracks in three other planes after last Friday's mid-air drama, in which a gaping hole suddenly appeared in the ceiling of a jet carrying 123 people.

"The FAA will issue an emergency directive tomorrow that will require operators of specific early Boeing 737 models to conduct initial and repetitive electromagnetic inspections for fatigue damage," the aviation watchdog said.

The action initially will apply to a total of about 175 aircraft worldwide, 80 of which are registered in the US, the FAA said.

Most of those are operated by Southwest Airlines, it added.

"Safety is our number-one priority," said Transportation Secretary Ray LaHood in the statement. "Last Friday's incident was very serious and could result in additional action depending on the outcome of the investigation."

None of the passengers or crew on Southwest's Flight 812 were seriously hurt when a hole tore open the fuselage on the 737-300 aircraft flying Friday from Phoenix, Arizona, to Sacramento, California.

The hole caused a sudden loss of cabin pressure, triggering a harrowing journey that saw passengers grasp for oxygen masks as the pilot brought the plane into a rapid descent.

The plane landed without incident at an Arizona military base with a five-foot by one-foot (1.5-meter by 0.3-meter) gash in its roof, just next to the overhead luggage bins used by passengers.

Southwest pulled 79 Boeing 737-300s out of service, cancelling hundreds of flights, to inspect them for "skin fatigue" -- and by Monday afternoon said three more planes had been found to have cracks in them.

The airline said it had completed checks on 67 of the withdrawn aircraft, 64 of which were okayed and returned to service.

"The airline expects to complete the inspections and be able to launch a full operation on Tuesday," it said.

The FAA order will require inspections using electromagnetic technology in specific areas of the fuselage on certain Boeing 737 aircraft in the -300, -400 and -500 series that have accumulated more than 30,000 flight cycles.

FAA administrator Randy Babbitt said the inspections were designed to detect cracking that cannot be spotted with visual inspection.

It will then require repetitive inspections at regular intervals.

"The FAA has comprehensive programs in place to protect commercial aircraft from structural damage as they age," he said.

Southwest runs an all-737 fleet of 548 Boeing aircraft, with an average age of 11.2 years. It includes about 170 Boeing 737-300s, the oldest in the fleet, produced from 1984 to 1999.

Boeing said it was working with the National Transportation Safety Board as it investigates the incident and the FAA, as well as assisting Southwest in its inspections.

Boeing spokesman Doug Alder said the US aircraft giant is preparing a service bulletin that will recommend lap-joint inspections on certain 737-300/400/500 series airplanes.

"This long-standing process, through which manufacturers, operators and regulators work together, helps ensure that the safety of the world's commercial jetliners continues at the highest levels," Alder said.

NTSB official Robert Sumwalt, briefing reporters on the ground in Arizona on Monday, said the incident had revealed a previously unknown area of potential weakness.

"It was not believed that this was an area that could fail," he said, adding that the investigation into the Southwest incident was likely to take about a year.