The United States plans to send about 1,400 more Marines to Afghanistan, a U.S. military official told CNN Thursday.

The Marines will support the expansion of its security efforts in southern Afghanistan, and they will be on the ground for only a few months, the official said.

"We intend to keep the pressure on the Taliban throughout the winter and take advantage of the security gains already achieved," the official said.

Southern regions such as Kandahar and Helmand provinces have long been major fronts in the war against the Taliban.