The Department of Defence will soon withdraw several hundred US active duty troops from the southern border with Mexico, Pentagon spokesperson Col. Rob Manning told reporters.

"Some units have completed their mission and they have already started to partially deploy," Manning said on Monday as quoted by the Hill. "Other units have been identified to rotate home and will be returning home over the next several weeks."

Manning added that about 5,200 troops are currently deployed on the southern border, which is already down from 5,900 service members.

On November 30, Defence Secretary James Mattis approved the Department of Homeland Security's request to extend the deployment of troops currently stationed along the US-Mexico border through the end of January. The original request was set to expire on December 15.

The active duty military personnel were deployed to the US southwest border in October to assist the Customs and Border Protection (CBP) upon the arrival of migrant caravans from Central America.