The FAA has given Thai aviation authorities a mid-October deadline to make corrective measures over shortcomings in its commercial aviation safety standards.
The regulator gave Thailand 65 days to address the issues from last Friday (July 17).
If it fails to meet the standards by the deadline, the FAA said Thailand-operated airlines would be banned launching new flight services to the US.
The ultimatum follows Thailand recently being awarded red flag status by ICAO over procedural and safety issues.
This then led to the FAA downgrading Thailand after finding three areas that needed fixing during a week-long inspection of its procedures.
It concluded there was a lack of qualified inspection staff; staff being inappropriately assigned to certain aircraft and lack of compliance with some standardised operations checklists.