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Flying between the United States and Mexico could get easier and more affordable.

In December, the two countries agreed to open their aviation markets to each other's carriers. Rules that had generally limited two or three airlines from each country to a particular route will go away.

American, Delta and Southwest have already announced that they will offer new flights across the border later this year. United is watching the demand for flights and will respond accordingly, a spokesman said.

Airlines on both sides of the border will be able to fly whatever routes they want as often as they want and set their own prices, said Thomas Engle, the State Department's deputy assistant secretary for transportation.

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"This will help reduce airfares for sure," said George Hobica, founder of the travel site airfarewatchdog.com.

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Hobica said base fares between the U.S. and many destinations in Mexico are already low, but both countries impose taxes that inflate the price of a ticket. "The fares are low, it's the rest that makes it seem expensive," he said.

For example, on a round trip between Dallas-Fort Worth and Cancun, Mexico — already a popular route — taxes and fees can account for between 20 and 30 percent of the price of a bargain, economy-class ticket of $383 to $585.

File photo: Passengers wait to get a seat on a Aeromexico airways's flight. GERARD JULIEN / AFP - Getty Images

Southwest promoted fares as low as $258 for a round trip, although seats were limited and the offer was scheduled to end Sunday.

The agreement between the U.S. and Mexico does not relax limits on takeoffs and landings at Mexico City's busy main international airport. So the first new flights from U.S. carriers will focus on resort towns in Mexico.

Delta Air Lines Inc. announced Friday that on Dec. 17 it will start daily nonstop flights between New York's Kennedy airport and Cancun and between Los Angeles and Los Cabos. It will run Saturday flights between Kansas City and Cancun.

Southwest Airlines Co. announced that on Dec. 4 it will start flying daily from Los Angeles to Cancun, Los Cabos and Puerto Vallarta. Southwest plans to fly from Oakland, California, to Los Cabos and Puerto Vallarta starting in February if it gets approval from the Mexican government.

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American Airlines Group Inc. will begin flying between Miami and Merida on the Yucatan peninsula on Nov. 4 and from Los Angeles to Cancun and Puerto Vallarta on Dec. 15, a spokesman said.

Engle, the State Department official, said in an interview that the agreement should help American travelers and increase Mexican tourism to the United States.

"We think it will help drive economic growth in sectors well beyond aviation, including tourism and manufacturing," he said.

The agreement also covers cargo airlines. It will let U.S. cargo carriers fly from airports in Mexico to other countries without stopping in the United States.

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