The fuselage of Boeing Co's upcoming 777X aircraft was split by a high-pressure rupture just as it approached its target stress level during a test in early September, Boeing said on Wednesday.

The world's largest planemaker suspended load testing of the new widebody in September when media reports said a cargo door failed a ground stress test. There have also been issues with General Electric Co's new GE9X turbine engine that will power the jet.

The Seattle Times, which first reported new details on the testing issue bit.ly/35AdjgE, said photos it had obtained of the test on the 777X showed that the extent of the damage was greater than previously disclosed and earlier reports were wrong about crucial details.

During the final load testing of a 777X test airplane, engineers ran a test that involved flexing the aircraft's wings beyond what is expected during normal commercial service, Boeing said.

"A testing issue occurred during the final minutes of the test, at approximately 99 percent of the final test loads, and involved a depressurization of the aft fuselage," Boeing said.

The company did not see any significant impact on the jetliner's design or preparations for first flight, and it did not see any impact from the test on the overall program schedule, Boeing said.

The 777X is due to fly for the first time in early 2020, with the first jet on track to be delivered to an airline in 2021, Boeing has said.

Boeing shares were down 0.6% at US$371.09.

Boeing said it is still assessing the root cause of the testing issue.

The U.S. planemaker would almost certainly not have to do a retest and regulators would likely allow Boeing to prove by analysis that it would be enough to reinforce the fuselage in the area where it failed, the Seattle Times said.

A spokesman for the Federal Aviation Administration, Lynn Lunsford, said the agency was "continuing our conversations with Boeing about the situation.

"The FAA requires manufacturers to meet design and certification standards. How they choose to do that is up to them," Lunsford said.

Tim Clark, president of Emirates - the largest customer for the 777X jet - has questioned the testing of the aircraft, telling reporters at last week's Dubai Airshow that he did not know when the Gulf airline would receive the first jet.