Dallas, Texas - Less than a week after launching service from Los Angeles International Airport to three Mexican cities, Southwest Airlines has canceled more than 30 of those flights since Wednesday due to an authorization issue with the Mexican government that has drawn a response from the U.S. Department of Transportation.

"We're still awaiting an official explanation to learn why these customary authorizations have not been issued, and we are optimistic that our close work with local authorities in Mexico will have us operating these newly added flights between Mexico and the U.S. very soon," Southwest spokesman Brad Hawkins said in an email.

The U.S. Department of Transportation is working "in close coordination with Southwest and our Mexican counterparts to resolve difficulties in operating to Mexico," a spokeswoman said Friday.

Southwest announced new flights from Los Angeles to Cancun, Los Cabos and Puerto Vallarta in August to take advantage of a recently approved bilateral treaty between the U.S. and Mexico that lifted restrictions on flights between the two countries.

Southwest's Los Angeles to Mexico routes were the first flights to take advantage of this new treaty when they launched December 4.

Southwest said it submitted all the required paperwork to Mexican aviation authorities more than three months ago and the routes are "fully compliant with terms of the bilateral aviation accord." But so far, the Mexican government hasn't issued the required authorization allowing Southwest to operate the flights.

Southwest inaugurated the service with a flight from Los Angeles to Puerto Vallarta on Sunday and continued operating flights Monday and Tuesday. But by Wednesday, the airline had canceled the 10 daily flights between Los Angeles and Mexico, with cancellations continuing at least through Saturday.

Southwest Airlines said it's reaching out to affected customers and working to reroute those wanting to continue their trips through its other international gateways in Houston, Denver and Orange County.