WASHINGTON — 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.

About 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 problems related to the cracks.

“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.

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 from 22,600 to 29,999 cycles must be inspected within 1,000 cycles,” the FAA said, adding that the planes will then have to follow up with a regimen of periodic inspections.

Southwest did not respond to questions on the company’s plans for handling the airplanes and inspections or whether it had discovered cracks.

American Airlines said that 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.”

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 from 22,600 to 29,999 cycles.