Boeing has discovered another issue with its grounded 737 MAX jet as it works to return the troubled aircraft to the skies this summer.

On Tuesday, the company confirmed that it found debris contaminating the fuel tanks in "several" undelivered planes built in the past year.

According to the Associated Press, Boeing said it has already made adjustments to prevent the problem from recurring in the future, including stepping up inspections before fuel tanks are sealed to ensure no metal shavings, tools or other objects are left behind during assembly, which poses an increased risk of electrical short-circuiting and fires.

FAA spokesman Lynn Lunsford told the AP that the agency has "increased its surveillance based on initial inspection reports and will take further action based on the findings."

General manager of Boeing's 737 program, Mark Jenks, called the discovery "absolutely unacceptable" in a memo to employees. However, the issue is not expected to delay the aircraft's timeline to return to service, with the 737 MAX's all-important certification flight expected to take place soon.

Boeing built approximately 400 undelivered MAX jets before temporarily suspending production in January. The aircraft has been grounded since last March following the second of two fatal crashes that killed a combined 346 people.

American Airlines, Southwest Airlines and United Airlines have each extended their 737 MAX cancellations into late-summer.