An F-35 fighter jet lost a two-foot-wide panel last week during a “routine training mission” off the Japanese coast, the Air Force said in a statement on Tuesday.

The panel, which was 12 inches by 24 inches, fell from the F-35A Lightning II on Thursday about 65 miles east of Okinawa. It was “most likely lost over the ocean,” according to a late Monday news release from 18th Wing Public Affairs.

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The aircraft was deployed to Kadena Air Base, Okinawa, south of the mainland.

“The item was discovered missing by the pilot’s wingman as the fighter jets were coming in for landing,” the release said. The service confirmed the panel was missing after a post-flight inspection.

The Air Force in early November sent 12 F-35A fighters and roughly 300 airmen and personnel to the base for a six-month rotational deployment, the first time the service’s version of the aircraft would be stationed in the country.

The Marine Corps since the beginning of the year has had its version, the F-35B, permanently stationed at its Air Station Iwakuni, on the south end of mainland Japan.

The release did not say whether the missing panel will cause issues in the way the aircraft operates, nor what type of training mission the F-35A was on.

“As a matter of policy, we do not discuss the details or specifics of our training or operations,” according to the statement.

The United States and South Korea are currently in the midst of a large-scale joint aerial drill, known as “Vigilant Ace,” which began Monday and will run through Friday. It includes F-22 stealth fighters and F-35 aircraft from the Air Force, Marine Corps and Navy, and comes after North Korea launched a new intercontinental ballistic missile last week following a two-month hiatus from such activities.