Southwest and Gol Linhas Aereas airlines have grounded 13 Boeing Co 737 NG planes after US regulators ordered urgent inspections on the models.

Concern over the Boeing jets developed after cracks were found in parts of the plane that connect the wings to the fuselage.

Southwest Airlines, based in the US, said two of the planes grounded were among a group of older jets that had to be inspected within seven days under an emergency order from the Federal Aviation Administration.

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Brazilian airline Gol, however, announced it had grounded 11 in total from their 115 Boeing 737 planes. According to Reuters, the airline said the fleet change would affect 3 per cent of its passengers between now and the end of the year.

Last week, the FAA ordered airlines to inspect Boeing 737s, starting with jets that have made at least 30,000 flights, after cracks were found in planes undergoing work in China.

"This condition could adversely affect the structural integrity of the aeroplane and result in loss of control of the aeroplane," FAA said in the order, which required jets with cracks to be grounded until they can be fixed.

Southwest spokesman Brian Parrish said crews inspected more than 200 planes and found signs of cracking on two, which will remain out of service until repairs are made.

The airline reported its findings to Boeing and the FAA, he said.

A Boeing spokesman said the manufacturer is working with airlines to develop repair plans and provide parts and technical help.

The cracks are in a part called a pickle fork because of prongs that extend under the wings. It had long been assumed that pickle forks would never need replacing.

According to Reuters, while minor aircraft cracks are not unusual, an industry source said it was slightly unusual for cracks to appear on the pickle fork before the end of an aeroplane's lifespan, which is more than 90,000 landings and takeoffs or "flight cycles".

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The FAA ordered that about 165 US planes be checked within seven days. A larger group, more than 1700 planes with at least 22,600 flights, must be checked before they fly another 1000 times.

The planes are a version of the Boeing 737 called the NG or next generation. Boeing is replacing the NG with the 737 MAX, but the MAX remains grounded worldwide after two crashes killed 346 people.

American Airlines also have the aircraft as part of their fleet, however none of the company's 737-800s "fall into the seven-day requirement".

These latest groundings for the Southwest 737 NGs come as the airline struggles with a grounded 737 MAX fleet.

Those groundings have led to the cancellations of an estimated 30,000 Southwest flights since March.

- AP