WASHINGTON (Reuters) - The Pentagon received a request on Friday from the Trump administration to extend its deployment of troops to the U.S. border with Mexico beyond a Dec. 15 authorization date to the end of January, officials said.

FILE PHOTO: U.S. Marines deploy concertina wire at the U.S. Mexico border in preparation for the arrival of a caravan of migrants at the San Ysidro border crossing in San Diego, California, U.S. November 15, 2018. REUTERS/Mike Blake

Defense Secretary Jim Mattis, who is expected to sign off on the extension of the mission, strongly hinted earlier this week that such a request by the U.S. Department of Homeland Security was in the works.

President Donald Trump ordered the deployment shortly before November congressional elections as a part of an effort to crack down on illegal immigration, as waves of thousands of migrants escaping violence in Central America trekked toward the United States.

Border security is a major issue among voters in Trump’s Republican Party.

Critics, including opposition Democrats in Congress but also some U.S. military veterans, have derided the troop deployment as a political stunt. Democrats have threatened to investigate the deployment once they take control in the House of Representatives next year.

The Department of Homeland Security, in a statement, cited the “the very real threat we face at the border from potential mass migration actions” when it confirmed the extension of the mission, which had been reported earlier on Friday by Reuters.

“The president has made it clear that border security is a top administration priority,” DHS spokeswoman Katie Waldman said in a statement.

This request refines support to ensure it remains aligned with the current situation, the nature of the mission, and Customs and Border Patrol operational requirements.

About 5,600 troops have been deployed to the border, but many of them have been involved in efforts to improve security around border crossings, including stringing up concertina wire. Many of those troops could be sent home. Remaining troops could focus on other missions, including helping fly U.S. border personnel to new positions along the border.

Still, officials caution that its unclear how far troop levels will decline. One U.S. official, speaking on condition of anonymity, said troop levels were not expected to decline dramatically.

Neither the Pentagon nor the DHS speculated about troop levels on Friday.

The Trump administration has justified the high-profile border mission on a perceived threat to the border, as thousands of migrants, mostly migrants from Honduras, flooded into the city of Tijuana across the border from San Diego, California, over the past several weeks.

U.S. customs and border control officers fired tear gas canisters into Mexico at dozens of migrants who tried to rush border fencing on Sunday.

Under the harsh immigration policies introduced by the Trump administration, U.S. border officials say the migrants may have to stay put in Mexico for months before they can petition authorities for asylum.