WASHINGTON — The Department of Homeland Security formally asked the Pentagon on Friday to extend the mission of active-duty troops on the southwest border through the end of January.

If Defense Secretary Jim Mattis approves the request, as one Pentagon official said was expected, at least some of the military forces will spend the Christmas holidays on the border.

In a statement, Katie Waldman, a Department of Homeland Security spokeswoman, said the request would ensure the military mission “remains aligned with the current situation” on the border, where the troops have been building barriers ahead of an arriving caravan of migrants from Central America.

The troops are also backing up Customs and Border Protection officers and Border Patrol agents who might need protection — even though most of the soldiers and Marines are not carrying guns.