WASHINGTON — The Pentagon is now actively planning for the U.S. troops deployed to the southern border to become a rotational force, meaning their mission would be extended and fresh troops would be rotated in, according to two U.S. officials and one former U.S. official familiar with the planning.

While there has not yet been a request for an extension from the Department of Homeland Security, defense officials are now planning for the mission to be extended past the Dec. 15 end date and for new troops to replace some of the ones currently serving there.

"There are discussions about extending the mission," confirmed a defense official, "but without a formal request, it may not actually happen. It still depends on if DHS wants to make the request or not."

"DOD has not received any formal request at this time," said Defense Department spokesperson Lt. Col. Jamie Davis said.

A source with direct knowledge of the discussions said the request to keep U.S. troops at the border longer will come from DHS to the Pentagon as early as this week. The official said one of the driving forces behind keeping troops on the border is another caravan coming up from Mexico in January.

"We are talking weeks or months, not years," according to one defense official, talking about how long the troops could be deployed to the border.

Another said the military is just conducting "prudent planning" by figuring out how to replace some of the current troops with new ones.

Many of the troops there now spent their Thanksgiving holiday along the border. Replacing them with new service members would mean many would not also spend Christmas away from home, but others would.

Last week Defense Secretary James Mattis opened up the possibility that the mission could be extended and U.S. troops could still be there over Christmas.

Speaking to a group of reporters last Wednesday, Mattis said, "Some of those troops certainly will be home. I would anticipate they would be. But some troops may not be, or some new troops may be assigned to new missions. But this is a dynamic situation."

There are about 5,600 U.S. troops currently assigned to the border mission.