The marines are from 24th Marine Expeditionary Unit, or MEU, which is doing combat operations in the volatile southern parts of Afghanistan.

The new order means they will come home in early November rather than October as the Pentagon promised earlier this year, according to David Nevers, a Pentagon spokesman.

"Their extension comes at the request of International Security Assistance Force, (ISAF) and with the approval of Defense Secretary Robert Gates," he said.

"This will afford the MEU the opportunity to continue building on the tremendous success they have achieved during their tour there," Nevers added.

The MEU was deployed to southern Afghanistan in March for what was to be a "one-time, seven-month" assignment promised by Pentagon Press Secretary Geoff Morrell at the time.