By Idrees Ali and Phil Stewart

WASHINGTON, Feb 2 (Reuters) - U.S. Defense Secretary Pete Hegseth's first trip since taking office will be to the United States' border with Mexico on Monday, in the latest sign that fortifying the border will be a priority for the Pentagon under President Donald Trump.

Trump has increasingly turned to the military to help carry out his immigration agenda, including sending additional troops to the border, using military aircraft to fly migrants out of the United States, and opening up military bases to help house them.

"POTUS wants 100% operational control of the border—and we will deliver," Hegseth said on Sunday on X, referring to Trump, as he announced the trip to visit troops on the border.

Trump declared a national emergency under the International Emergency Economic Powers Act on Saturday, citing the "extraordinary threat" from fentanyl and illegal immigration, and imposed tariffs on Mexico, Canada and an extra duty on Chinese goods.

Republican Trump last week said he was expanding a detention facility at the U.S. naval base in Guantanamo Bay, Cuba, to hold 30,000 people. His White House border czar, Tom Homan, has said he hopes to start moving migrants there within 30 days.

Additional U.S. Marines arrived at Guantanamo Bay in recent days to prepare to expand a facility that holds migrants.

The Pentagon has also started providing flights for the deportations of more than 5,000 immigrants held by U.S. authorities in El Paso, Texas, and San Diego, California.

Two U.S. officials, speaking on the condition of anonymity, said that U.S. military aircraft flew detained migrants to Honduras and Peru over the weekend.

The military flights are a costly way to fly migrants. Reuters reported that a military deportation flight to Guatemala last week likely cost at least $4,675 per migrant.

That is more than five times the $853 cost of a one-way first-class ticket on American Airlines from El Paso, Texas, the departure point for the flight.

(Reporting by Idrees Ali and Phil Stewart; Editing by Saad Sayeed)

((Idrees.Ali@thomsonreuters.com; 301-747-8263;))