The Department of Homeland Security (DHS) has asked the Pentagon to extend the deployment of roughly 6,000 active-duty troops at the southern border with Mexico over a month, to Jan. 31, 2019, Fox News has learned.

It is not immediately clear if Defense Secretary James Mattis has approved the request. The previous deployment orders have a Dec. 15 expiration. If Mattis does approve the request, thousands of soldiers who had planned to be home for Christmas will continue their deployment away from home.

"This request refines support to ensure it remains aligned with the current threat, the nature of the mission and CBP (Customs and Border Patrol) operational requirements," Lt. Col. Jamie Davis, a Defense Department spokesperson, said in a statement.

About 2,800 of the active-duty troops are in South Texas, far from a caravan of Central American migrants in Tijuana, Mexico, south of California. The movement of the migrants into Mexico last month was the stated reason that President Trump ordered the military to provide support for CBP.

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Fox News' Sam Chamberlain contributed to this report.