The U.S. Army is preparing to launching a major offensive against a Taliban stronghold in Afghanistan.

About 400 U.S. troops are set to attack Marjah, the largest Taliban controlled town in southern Afghanistan.

Officials have not said when the main attack will take place, but troops are just north of the city preparing for the offensive.

Demands that militants disarm have been sent, and residents have been warned to stay inside and avoid road travel once the operation begins.

"The message to the people of the area is of course keep your heads down, stay inside when the operation is going ahead," NATO's civilian chief in Afghanistan, former British Ambassador Mark Sedwill, said.

This is the first major offensive since President Obama sent in an additional 30,000 troops.