US planemaker Boeing admitted on Thursday that its 737 MAX passenger jet has another software error in addition to the one implicated in two major crashes in recent months.

Boeing said the problem, which it described as a "relatively minor issue" that was already solved, had become apparent during work on overhauling the controversial anti-stalling system MCAS, but was not directly connected with it.

The manufacturer's statement comes after the US daily "Washington Post" reported that the US aviation regulator FAA had made the correction of the second problem another precondition for the aircraft to be allowed to fly again. The newspaper cited sources familiar with the FAA investigation as saying that the problem could affect aircraft safety.

All 737 MAX airplanes have been grounded since mid-March following the Ethiopian Airlines crash of March 10, in which 157 people were killed.

Read more: Boeing 737 MAX: Is the new airplane safe?

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The MCAS (Maneuvering Characteristics Augmentation System) is suspected of being responsible both for the Ethiopian Airlines crash and one of a 737 MAX operated by Indonesian airline Lion Air in October last year, in which all 189 passengers and crew were killed.

A preliminary report on the Ethiopian Airlines crash released on Thursday did not directly blame the MCAS, but did speak of the plane's constant and uncontrollable nose-diving, which could have been caused by the software if it were fed false data from a damaged sensor.

Boeing CEO Dennis Muilenburg said in a statement Thursday that his company was working on an update to the MCAS "that will ensure accidents like that of Lion Air Flight 610 and Ethiopian Airlines Flight 302 never happen again." He said he expected the update to be certified and implemented worldwide in the next few weeks.

Despite the controversy surrounding the 737 MAX, Boeing's stock rose on Thursday by 1.3 percent after falling some 6 percent following the Ethiopian Airlines crash.

Read more: Why do airplanes stall and why is it so dangerous?

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