Southwest said it removed the two planes from its operation. (Representational)

Southwest Airlines Co said Wednesday it found cracks on two Boeing 737 NG airplanes and removed them from service after US regulators last week ordered urgent inspections.

The US Federal Aviation Administration last week told aircraft operators to inspect 165 Boeing 737 NG airliners for structural cracks within seven days after the issue was found on a small number of planes. Southwest, which did not find any issues in the "vast majority" of the planes, said it "removed the two aircraft from our operation and reported the findings to Boeing and the FAA."

The aircraft will remain out of our schedule until the issues have been fully resolved."Nearly all the 165 planes are Southwest aircraft," officials said.

Last week, the FAA said planes with fewer flights will eventually get inspected. The order covers a total of 1,911 US registered planes.

The inspections can be done visually and take about an hour per aeroplane. The cracks are on what is known as the "pickle fork" -- a part that attaches the plane's fuselage, or body, to the wing structure and manages forces. The inspections look for "cracking of the left and right hand side outboard chords of frame fittings and failsafe straps," the FAA said last week, warning the issue "could adversely affect the structural integrity of the aeroplane and result in loss of control of the aeroplane."

Southwest, United Airlines and American Airlines Inc are among carriers that fly the 737 NG. The NG includes the 737-600, -700,-700C,-800, -900, and -900ER. American Airlines said none of the company's 737-800 fleet "fall into the seven day requirement."

The company anticipates around 80 737-800 aircraft will require this inspection in the next eight months.The 737 NG is the third-generation 737 and the version before the now-grounded 737 MAX, which is not impacted by the issue.

Boeing said earlier it has been in contact with 737 NG operators about the issue, but added that "no in-service issues have been reported." Aircraft with more than 30,000 cycles must be inspected within seven days, while planes between 22,600 and 29,999 cycles must be inspected within 1,000 cycles, which typically correspond to the number of flights.

United said it also does not fall under the 7-day time frame and 80 of its airplanes are "between the 22,600 and 29,999 cycles" and will be inspected as required.