WASHINGTON — The Department of Homeland Security asked the Pentagon on Friday for a 45-day extension of the U.S. military presence at the Mexico border, a request that would stretch the deployment until at least the end of January.

The Defense Department is expected to agree to the extension in the coming days, well ahead of the mission’s current expiration date, which is Dec. 15.

Pentagon officials have said that some of the 6,000 active-duty personnel stationed along the border in Arizona, California and Texas would be brought home and replaced by other units.

President Donald Trump ordered the deployment to pre-empt the arrival of thousands of Central American migrants traveling in caravan groups and seeking to enter the country. His administration has characterized the migrants, who have concentrated along Mexico’s border with California, as a grave security threat.

“Given the ongoing threat at our Southern border — today the Department of Homeland Security submitted a request for assistance to the Department of Defense to extend its support through January 31, 2019,” 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 Protection) operational requirements,” she said.

A Pentagon spokesman, Lt. Col. Jamie Davis, confirmed the request.

Homeland Security Secretary Kirstjen Nielsen has also sent a memo to the departments of Justice, State and Interior, among others, requesting backup at the border from their law enforcement personnel.

The unusual request, first reported Friday by Politico, would potentially deputize forest rangers, U.S. marshals and other federal officers to work as auxiliary immigration agents.

A Justice Department official confirmed Friday that the Drug Enforcement Administration would provide 33 agents, and that the U.S. Marshals service is sending 10 of its staff to the border in California. Their role was not clear, but an official said they would lend assistance at the request of CBP.

U.S. marshals and DEA agents already have a large and active presence along the Mexico border as part of their routine law enforcement duties.

“If the need arises for their services, they’ll be there,” the Justice Department official said. “The caravan presents a unique situation.”

DHS officials said Nielsen’s request did not indicate how many law enforcement officers the agency is seeking, but described the call for help as a unremarkable measure.

In Tijuana, Mexico, authorities have begun moving Central American migrants from a crowded, increasingly unsanitary encampment to a government-run shelter farther from the U.S.-Mexico border, raising fears among members of a migrant caravan that their dreams of asylum in the United States will be dashed.

But many migrants, the bulk of them from Honduras, were staying put, suspicious of Mexican authorities' intentions.