Southwest Airlines has stopped new flights between Los Angeles and Mexico because Mexican authorities haven't finished the paperwork formally authorizing the service.
American Airlines has been forced to delay one new route to Mexico while it too awaits approval from Mexico.
In both cases, the new flights were added after the U.S. and Mexico agreed in August to loosen restrictions that had limited cross-border flights.
The Southwest cancelations cover 10 daily flights between Los Angeles and the Mexican resort cities of Los Cabos, Puerto Vallarta and Cancun. Southwest started flying the routes on Sunday but then suspended service after Tuesday's flights, according to data from tracking service FlightAware.com.
Southwest submitted all required paperwork to Mexican authorities more than three months ago, airline spokeswoman Brandy King said Friday.
"Because required paperwork still has not been issued by authorities in Mexico, we are now faced with unplanned challenges and forced to make proactive flight cancellations," King said by email.
A spokeswoman for the U.S. Department of Transportation confirmed that the government has gotten involved to resolve the issue, but she declined to comment further. Mexican aviation authorities had no immediate comment when contacted by The Associated Press.
Dallas-based Southwest said that its other, older routes between the U.S. and Mexico are operating normally.
After the August accord, American Airlines announced that it would launch new service between Miami and Merida, Mexico, on Nov. 4. That service has been delayed because Republic Airlines, which was to operate the flights for American, submitted paperwork but has not yet received approval from Mexico, said American spokesman Matt Miller.
However, American did receive OK for new flights between Los Angeles and both Puerto Vallarta and Cancun, and expects to start those routes on schedule next week, Miller said.