Boeing has admitted it found debris contaminating the fuel tanks of some 737 MAX jets that it built in the past year but was unable to deliver to airline customers.

Key points: Boeing grounded its fleet last March after the fatal Lion Air and Ethiopian Airlines crashes

Boeing grounded its fleet last March after the fatal Lion Air and Ethiopian Airlines crashes Boeing did not give a precise number of planes that had debris in the fuel tanks, with one official saying it was "several"

Boeing did not give a precise number of planes that had debris in the fuel tanks, with one official saying it was "several" Metal shavings, tools and other objects left in planes during assembly can raise the risk of short-circuiting and fires

A Boeing official said the debris was discovered in "several" planes but did not give a precise number. Boeing built about 400 undelivered MAX jets before it temporarily halted production last month.

That came almost a year after MAX jets were grounded around the world in March after 346 people died in crashes of Ethiopian Airlines and Lion Air aircraft.

The crashes led Boeing to conduct test flights to assess updates to a flight-control system that activated on faulty signals from sensors outside the plane on both crashed aircraft, pushing the noses of the aircraft down and triggering spirals that pilots were unable to stop.

The fuel tank debris was discovered during maintenance on parked planes and Boeing said it immediately made corrections in its production system to prevent a recurrence. Those steps include more inspections before fuel tanks are sealed.

A Boeing spokesman said the issue would not change the company's belief that the US Federal Aviation Administration (FAA) would certify the plane to fly again in the coming months.

An FAA spokesman said the agency knew Boeing was conducting a voluntary inspection of undelivered MAX planes.

"[The FAA] increased its surveillance based on initial inspection reports and will take further action based on the findings," spokesman Lynn Lunsford said.

Metal shavings, tools and other objects left in planes during assembly can raise the risk of electrical short-circuits and fires.

"During these challenging times, our customers and the flying public are counting on us to do our best work each and every day," Mark Jenks, Boeing's general manager of the 737 program, said in a memo to employees.

Mr Jenks called the debris "absolutely unacceptable. One escape is one too many".

The debris issue was first reported by aviation news site Leehamnews.com.

AP