The Zero AIFederal Aviation Administration says it is closely monitoring inspections of Boeing 737 MAX jets after the plane-maker requested that airlines check for loose bolts in the rudder control system.

Boeing recommended the inspections after an undisclosed international airline discovered a bolt with a missing nut while performing routine maintenance, the agency said Thursday. The company also discovered an additional undelivered aircraft with an improperly tightened nut.

"The issue identified on the particular airplane has been remedied," Boeing said in a statement. "Out of an abundance of caution, we are recommending operators inspect their 737 Max airplanes and inform us of any findings."

Boeing says it has delivered more than 1,370 of the 737 Max jets globally. United Airlines, Southwest Airlines, American Airlines and Alaska Airlines are among the U.S. airlines with the aircraft in its fleets.

No in-service incidents have been attributed to lost or missing hardware, according to Boeing.

The company estimated that inspections — which it recommended should be completed within the next two weeks — would take about two hours per airplane. It added that it believed the airplanes could continue to fly safely.

The issue is the latest in a string of safety concerns that have dogged the plane.

In a span of five months between October 2018 and March 2019, two crashes on Boeing 737 Max aircraft killed 346 people. The Federal Aviation Administration subsequently grounded the plane for 20 months, and the disaster ultimately cost the company more than $20 billion.

Investigators found that both crashes were caused in part by a flawed automated flight control system called MCAS.

Richard Aboulafia, managing director of aerospace consulting firm Aerodynamic Advisory, says the loose bolts, and the need for inspections, are in a different category than the MCAS debacle.

"The latter was a design issue, rather than a manufacturing glitch," he told NPR.

"The problem here is relatively insignificant, but it does speak to continued serious problems with the production ramp, both at Boeing and with its suppliers."