The Federal Aviation Administration's (FAA) acting agency head announced on Thursday that officials did not expect to meet a key deadline to certify the Boeing 737 MAX 7.
The deadline was to be met before the end of the year, Reuters reported.
FAA said in October that some submissions were incomplete in the review of the aircraft.
The agency reported that some key documents that were submitted in an ongoing investigation into the plane, but said some were not complete and that others needed a reassessment by the U.S. aircraft manufacturer.