Tests on Boeing's grounded 737 MAX have revealed a new safety risk unrelated to the two fatal crashes that led to the grounding of the aircraft. US regulators are ordering the company now to make additional design changes.

The Federal Aviation Administration discovered that data processing by a flight computer on the jetliner could cause the plane to dive in a way that pilots had difficulty recovering from in simulator tests, according to two people familiar with the finding who asked not to be named to discuss it.

"The FAA recently found a potential risk that Boeing must mitigate," the agency said in an emailed statement on Wednesday. The statement didn't provide any specifics.

Boeing's troubled 737 MAX has been grounded since March in response to the fatal crashes. Credit:AP

While the issue didn't involve the Maneuvering Characteristics Augmentation System linked to the two fatal accidents since October that killed 346 people, it could produce an uncommanded dive similar to what occurred in the crashes, according to a source who wasn't authorised to speak about the matter.