Boeing was hit by new safety fears on Tuesday after the company admitted that it had found debris inside the fuel tanks of some of its 737 Max jets.

The 'foreign objects' were found in the tanks of the planes that have been grounded at Boeing's production plant after the 737 MAX 8 was grounded last year following two deadly crashes that killed 346 people.

The planes were grounded after investigators found that the model's new flight software and a lack of training for pilots confused pilots and led to the crash of Lion Air flight 610 in Indonesia and Ethiopian Airlines flight 302.

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

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.

In this 11 December, 2019, file photo, a United Airlines Boeing 737 Max airplane takes off in the rain at Renton Municipal Airport in Renton, Wash.

Rescuers work at the scene of an Ethiopian Airlines flight crash in March 2019 which caused Boeing to ground its popular 737 Max aircraft to investigate its link with the crash

A Boeing spokesman said that the issue would not change the company´s belief that the Federal Aviation Administration will certify the plane to fly again this summer.

An FAA spokesman said the agency knows that Boeing is 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,' said spokesman Lynn Lunsford.

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

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

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

The Federal Aviation Administration at 600 Independence Avenue in Washington,DC - messages released by Boeing appear to show employees deceiving the FAA over safety issues

A graphic shows the dimensions of the Boeing's 737 Max model

A graphic shows Boeing's 737 Max maneuvering characteristics augmentation system

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

Max jets were grounded around the world last March after two crashes killed 346 people.

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

While investigators examining the Max accidents have not pointed to production problems at the assembly plant near Seattle, Boeing has faced concerns about debris left in other finished planes including the 787 Dreamliner, which is built in South Carolina.