The US' Federal Aviation Administration (FAA) has said that Boeing's 737 MAX jets will remain grounded for weeks as the plane maker is continuing to work on a software fix.
Boeing had said it would submit the fix to the FAA last week, and it had gathered hundreds of industry representatives at its Seattle-area facilities on March 27 to demonstrate the software changes, CNN reported.
But the FAA said on Monday that the company concluded that "additional work" was needed.
"The FAA expects to receive Boeing's final package of its software enhancement over the coming weeks for FAA approval," the agency said in a statement.
"Time is needed for additional work by Boeing as the result of an ongoing review of the 737 MAX Flight Control System to ensure that Boeing has identified and appropriately addressed all pertinent issues."
Boeing acknowledged the new timeline, saying in a statement that the software change would be "completed in the coming weeks". It did not say why the timeline had changed.
"Safety is our first priority, and we will take a thorough and methodical approach to the development and testing of the update to ensure we take the time to get it right," the company said.
An aviation source told CNN that Boeing had discovered "an additional piece to address" in the final review of the new software and that integrating new flight control software on a complex aircraft takes time.
Prior to the discovery, Boeing had expected the new software to be ready within days.
American Airlines said on Monday that it was aware that the MAX "may be further delayed" in returning to service. It has been forced to cancel dozens of flights daily while the planes await a fix.
American operates 24 MAX 8 airplanes and Southwest operates 34. United Airlines operates 14 of the larger 737 MAX 9 jets.
The FAA followed other countries' aviation authorities in grounding the MAX on March 13 following the second crash in less than five months.
The agency said it had reviewed satellite data and physical evidence that showed similarities between the Lion Air flight that crashed in October and the Ethiopian Airlines flight that crashed in March 13.
A preliminary report on the Ethiopian accident is expected in the coming days.
The FAA also said on Monday that the software changes would require a "rigorous safety review".
"The FAA will not approve the software for installation until the FAA is satisfied with the submission," it added.