More than three dozen Boeing 737s have been grounded worldwide after wing-related cracks were discovered during recent inspections, the company said Thursday.

That includes a small number of U.S. planes, the Federal Aviation Administration said. Southwest reported that it had found two cases of the cracks among its planes with the highest number of uses, which an urgent FAA order Oct. 3 required to be inspected within seven days.

Boeing last month notified the FAA that cracks had been discovered in a critical structural component of Next-Generation 737s that helps attach the wings and the main body of the planes. The problem “could adversely affect the structural integrity of the airplane and result in loss of control of the airplane,” the FAA said.

Boeing said its customers have completed 810 inspections so far, finding 38 cases that will “require repair and replacement of the affected parts.”

“Our technical team is working to determine the most efficient repair plan for customers that ensures the safety, quality and integrity of the airplane. We are investigating the root cause of the issue,” Boeing said in a statement.

Boeing said the cracking does not affect any of its 737 Max aircraft, which remain grounded after investigators said a flawed automated anti-stall system contributed to two deadly crashes within five months.

The FAA is working with Boeing and international aviation safety regulators “to better understand the factors that led to the formation of the cracks” in the 737 NG jets, the safety agency said. “The FAA will continue to monitor the situation and will consider additional action as necessary.”

Southwest said that “we did not find abnormalities on the vast majority of our inspected fleet.” The grounded aircraft “will remain out of our schedule until the issues have been fully resolved,” the airline said, and its technicians are “now focused on completing inspections of the remaining portion of the 737 NG fleet covered by the” FAA order.

Internationally, Brazilian low-cost airline GOL said it had grounded 11 planes after discovering cracks. It said it “deeply regrets any inconvenience” caused to customers who had to be booked on other planes.

Boeing said safety and quality are its top priorities.

“Boeing regrets the impact this issue is having on our 737NG customers worldwide,” the company said.

The FAA inspection order is based on how many “cycles” a plane has experienced, meaning how many times it has gone through pressurization and depressurization. The order has longer inspection deadlines for planes with fewer cycles.

American Airlines spokesman Ross Feinstein said that while “we don’t have aircraft that require immediate inspection” under the FAA order, the company has examined four aircraft thus far and found no issues.

United said its aircraft “will be inspected within the required time frame.”