For decades, Southwest Airlines flew only domestic flights, so it could keep its operation simple, reliable and lucrative.

International service can be profitable, too, but it requires airlines to obtain all sorts of regulatory approval from foreign governments.

Southwest is now learning, the hard way, how challenging it can be.

[UPDATED: Southwest Airlines says it has now received approval to fly to Mexico.]

The airline said this week it is being forced to cancel three new flights from Los Angeles to Mexico for the foreseeable future, due to regulatory restrictions. The new flights — to Cancun, Los Cabos, and Puerto Vallarta — started with great fanfare on Dec. 4, and lasted three days before regulators realized something was amiss.

In a statement, Southwest was coy about exactly what the problem is, blaming the issue on “paperwork.”

“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,” the statement said.

Southwest is among the first airlines to take advantage of a new bilateral agreement between the U.S. and Mexico that allows more carriers to serve more routes. In the past, only three U.S. airlines could fly to Mexican beach cities from Los Angeles, and Southwest, as a new entrant, had been shut out.

In its statement, Southwest said it thought it was doing everything required to begin new service, under the terms of the bilateral agreement.

“Southwest became the first carrier to announce, sell, and operate new flights under a bilateral agreement of aviation protocols enacted last August and designed to bring additional flights and low-fare service between the U.S. and Mexico,” the airline said. “We are fully compliant with the terms of that bilateral aviation accord, having submitted all required regulatory paperwork to Mexican authorities more than three months ago in order to begin new service [on] Dec. 4, 2016.”

American Airlines will start new flights from Los Angeles to Cancun and Puerto Vallarta on Dec. 15, and the carrier already has all the regulatory approvals it needs, airline spokesman Matt Miller said.

American is having similar trouble with a new American Eagle flight from Miami to Merida, Mexico, Miller said. However, unlike Southwest, Republic Airways, the operating carrier, never started flying the route. It will only do so once it receives final approval, Miller said.

A consultant who regularly helps airlines navigate regulatory issues in Latin America said it rare for a carrier to start flying a route before receiving final approvals.

“At carriers I have done work with they have detailed checklist of what regulatory items need to be applied for and received prior to commencement,” the consultant said. “It could very well be that Southwest does not know what it does not know yet as this is a new world for it.”

All of Southwest’s other Mexico flights are operating as normal, as they were cleared by the old U.S.-Mexico agreement. That includes flights from nearby Santa Ana, California.

“We understand the U.S. Department of Transportation is actively engaged at the highest levels of its counterpart agency in Mexico to resolve the situation,” Southwest said. “We continue our discussions and work with Mexican authorities to understand why the paperwork has not been issued in the way agreed to in the bilateral aviation accord.”

Southwest said it is contacting passengers to offer them flight alternatives or refunds.

Photo Credit: Southwest failed to get final clearance from the Mexican government before starting three new Mexico routes. LM Otero / Associated Press