Southwest Airlines Co and Brazil's Gol Linhas Aereas have grounded a total of 13 Boeing Co 737 NG airplanes, the companies said, after U.S. regulators ordered urgent inspections.

Southwest said it had taken two planes out of service, while Gol had grounded 11. Both airlines are major operators of the 737, including the NG variant and the more recent MAX, which has been grounded for months after two deadly crashes.

The U.S. Federal Aviation Administration last week told aircraft operators to inspect 165 older 737 NG planes for structural cracks within seven days, after cracking was found on a small number of planes. Nearly all the 165 jets were Southwest aircraft, officials said.

First delivered in 1997, the NG is the third generation of the 737, preceding the grounded MAX, which is unaffected by the cracking issues.

Southwest, which did not find any issues in the "vast majority" of planes, said it "removed the two aircraft from our operation and reported the findings to Boeing and the FAA."

Gol, which runs a fleet of 115 737s excluding its grounded MAX jets, said it took 11 out of service after finding "evidence of the need to replace a specific component." The fleet change will affect 3% of passengers until Dec. 15, it said.

The inspections were ordered following the discovery of cracks on a part known as the "pickle fork," which attaches the fuselage to the wing structure and manages forces.

Minor aircraft cracks are not unusual, an industry source said, but are not normally expected to occur on the pickle fork until near the end of the plane's lifespan, typically considered to be more than 90,000 takeoff and landing cycles.

The FAA mandated checks for "cracking of the left and right hand side outboard chords of frame fittings and failsafe straps," warning the issue "could adversely affect the structural integrity of the airplane" and result in loss of control. More planes will eventually be inspected, it said.

The initial inspection order covered a total of 1,911 U.S.-registered planes. The checks can be done visually and take about an hour per airplane.

Aircraft that have carried out more than 30,000 flight legs, or cycles, must be checked within seven days, the safety regulator said, while those with 22,600-29,999 cycles must be checked before completing a further 1,000.

Boeing has been in touch with 737 NG operators about the inspections, the manufacturer said earlier, emphasizing that "no in-service issues have been reported."

United Airlines and American Airlines Inc are among other U.S. carriers flying the 737 NG, which includes the 737-600, -700, -700C, -800, -900, and -900ER variants.

Each airline has about 80 planes that will need inspection in coming months, United and American said, but none that fall into the seven-day requirement for more urgent checks.

European budget carrier Ryanair, which has a fleet of more than 450 737-800s, confirmed it was in the process of carrying out the inspections, without saying how many planes required them.

"We are midway through the first part of this mandatory check program and don't expect it will have any impact upon our fleet or operations," a Ryanair spokeswoman said.

Transavia Airlines, a low-cost carrier owned by Air France-KLM, said it would examine all 42 of its 737s even though only 16 need checks under the FAA mandate - of which one was inspected under the seven-day requirement. KLM, which also operates 737 NGs, has inspected all but one of 13 aircraft covered by the order and found no anomalies, a spokesman said.

Jeju Air, a South Korean low-cost operator, had also completed the majority of required checks, while inspections were ongoing at domestic rival Jin Air and had been completed at Eastar Jet. None of the three airlines had found any issues, they said.