The Federal Aviation Administration is ordering airlines to inspect key structural components on more than 1,900 Boeing airplanes to look for cracks that could endanger the aircraft.

Southwest, United, and American are among the U.S. airlines that fly the Next-Generation 737 jets covered under the urgent order, which will be formally issued Thursday. More than 165 U.S.-registered planes will have to be inspected within seven days, the FAA said Wednesday.

“Boeing notified the agency of the matter after it discovered structural cracks on an aircraft undergoing modifications in China,” the FAA said in a statement. “Subsequent inspections yielded similar cracks in a small number of additional planes.”

The cracks were discovered in a joint and “fail-safe” parts related to support beams in the jets’ wings, according to the FAA and Boeing.

The problem “could adversely affect the structural integrity of the airplane and result in loss of control of the airplane,” according to a draft of the FAA order published Wednesday on the website of the Federal Register.

Boeing said no airlines have reported safety incidents related to the cracks.

“We are actively engaged with and supporting our customers … and have provided detailed instructions for conducting the inspections and reporting the results,” Boeing said in a statement, adding that “safety and quality are our top priorities.”

“This issue does not affect any 737 Max airplanes or the P-8 Poseidon,” a derivative of the 737 NG designed for anti-submarine warfare and surveillance, the company said.

Boeing has come under scrutiny over its management of airplane safety, design and quality issues after two new 737 Max aircraft crashed within five months of each other in Indonesia and Ethi­o­pia, killing more than 300 people. The National Transportation Safety Board reported last week that Boeing had underestimated the risk of — and overestimated pilots’ ability to handle problems with — an automated feature that contributed to the crashes.

Years before those crashes, the FAA found a pattern of recurring safety problems with Boeing. U.S. regulators found, for example, that a Boeing subcontractor had falsified certifications on cargo doors; that Boeing mechanics left tools inside plane wings close to the cables that control their movements; and improperly installed wires on 787s, which could increase the risk of shorts or fires, among other issues.

In this case, it is not clear what caused the cracks, how widespread the problem may be, and whether it is a reflection of any broader safety issues with the company.

The FAA said it was issuing its order because the agency “determined the unsafe condition. . . is likely to exist or develop in other products of the same type design.”

Airlines are required to report their findings from the inspections, which “will enable the manufacturer to obtain better insight into the nature, cause, and extent of the cracking, and eventually to develop final action to address the unsafe condition,” the FAA said.

The FAA’s order applies to planes that have reached certain thresholds for time in the air. It considers the number of “cycles” an airplane has been through, meaning how many times it has gone through pressurization and depressurization. “Cycles typically correspond to the number of flights and are not dependent on the calendar age of an aircraft,” the FAA said.

The order “will require operators to inspect aircraft with more than 30,000 cycles within seven days of the issuance of the rule. Aircraft between 22,600 and 29,999 cycles must be inspected within 1,000 cycles,” the FAA said, adding that airlines will then have to follow up with a regimen of periodic inspections.

Southwest, which flies only 737s, said it began the inspection process Tuesday.

“To date, Southwest has not had any unusual findings associated with the pickle fork on our” aircraft, the company said in a statement, using another name for the component at issue.

Southwest did not answer a question on how many of its planes are covered by the order, but said, “our Technical Operations Team has developed a schedule to perform overnight inspections on each of the identified aircraft and, based upon that schedule, we do not anticipate any disruption to our operation.”

In a statement, American Airlines said it was working closely with the FAA and Boeing on the new inspection requirements, and that “none of American’s aircraft in the 737 fleet fall into the seven-day requirement.”

American said its oldest Next-Generation 737 was delivered in 1999 and has about 25,000 cycles. With an average of roughly 1,200 cycles to 1,500 cycles a year, the company said, it anticipates it will have to perform about 80 of the inspections over the next eight months. The company said it expects no impact on its operations.

United also said none of its 737 NG aircraft are covered by the seven-day deadline, and the company anticipates about 80 planes will be subject to the inspection requirements for planes with between 22,600 and 29,999 cycles. The inspections will not have any impact on the company’s operations, it said, adding: “safety is our top priority at United Airlines.”