The Trump administration imposed new restrictions Saturday on flights from Mexico and threatened to end a longstanding partnership between Delta Air Lines and Aeromexico in response to limits the Mexican government placed on passenger and cargo flights into Mexico City several years ago.

Transportation Secretary Sean Duffy said Mexico’s actions to force airlines to move out of the main Benito Juarez International Airport to the newer Felipe Angeles International Airport more than 30 miles (48.28 kilometers) away violated a trade agreement between the two countries and gave domestic airlines an unfair advantage. Mexico is the top foreign destination for Americans with more than 40 million passengers flying there last year.

“Joe Biden and Pete Buttigieg deliberately allowed Mexico to break our bilateral aviation agreement,” Duffy said, referring to the previous president and his transportation secretary. “That ends today. Let these actions serve as a warning to any country who thinks it can take advantage of the U.S., our carriers, and our market. America First means fighting for the fundamental principle of fairness.”

All Mexican passenger, cargo and charter airlines will now be required to submit their schedules to the Transportation Department and seek government approval of their flights until Duffy is satisfied with the way Mexico is treating U.S. airlines.

It’s not immediately clear how Duffy’s actions might affect the broader trade war with Mexico and negotiations over tariffs. A spokesperson for Mexico’s President Claudia Sheinbaum didn’t reply immediately to a request for comment. Sheinbaum didn’t mention the new restrictions during either of her two speaking events on Saturday.

Delta and Aeromexico have been fighting the Transportation Department’s efforts to end their partnership that began in 2016 since early last year. The airlines have argued that it’s not fair to punish them for the Mexican government’s actions, and they said ending their agreement would jeopardize nearly two dozen routes and $800 million in benefits to both countries’ economies that come from tourism spending and jobs.

“The U.S. Department of Transportation’s tentative proposal to terminate its approval of the strategic and pro-competitive partnership between Delta and Aeromexico would cause significant harm to consumers traveling between the U.S. and Mexico, as well as U.S. jobs, communities, and transborder competition,” Delta said in a statement.

Aeromexico’s press office said it was reviewing the order and intended to present a joint response with Delta in the coming days.

But the order terminating approval of the agreement between the airlines wouldn’t take effect until October, and the airlines are likely to continue fighting that decision.

The airlines said in a previous filing fighting the order that it believes the loss of direct flights would prompt over 140,000 American tourists and nearly 90,000 Mexican tourists not to visit the other country and hurt the economies of both countries with the loss of their spending.

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Associated Press writer Amaranta Marentes in Mexico City contributed to this report.