Boeing has warned airlines about potential flaws on the wings of some 737 aircraft, including on the new-generation 737 Max that was grounded after two crashes.
The company has identified possible faulty parts on more than 300 aircraft worldwide.
The parts, called wing slats, help reduce drag on take-off and landing.
Affected parts may be susceptible to premature failure or cracks, the US aviation regulator said.
The Federal Aviation Administration Authority (FAA) added that the issue arose due to an “improper manufacturing process”.
The president and chief executive of Boeing Commercial Airplanes, Kevin McAllister, said in a statement: “We are committed to supporting our customers in every way possible as they identify and replace these potentially non-conforming tracks.”
News of the potentially faulty wing parts prompted a 1.6% fall in Boeing’s share price at the start of trading, and comes as the plane-maker grapples with the consequences of the 737 Max grounding.
The planes were grounded worldwide after a combined 346 people died in two crashes, the first in Indonesia in October followed by one in Ethiopia in March.
That also knocked the reputation of the FAA amid questions over its oversight during the flight certification process of the Max.
Boeing is working on a software fix that will allow the Max to begin flying again, but differences have arisen between the US and Canada on how to train pilots on the software after the update.
Washington believes training on computers or tablets is sufficient for seasoned pilots, but Ottawa wants to require training on flight simulators.
Other regulators around the world are trying to work on a coordinated plan to get the Max back into the skies.