The US would send more troops to Afghanistan to improve security, which has declined sharply over the past few years, US Secretary of Defence Robert Gates said during a surprise visit to US soldiers in the southern province of Kandahar.
A brigade would be deployed next month, and another two brigades of 3,500 soldiers each would be sent into the region, he was quoted as saying by CNN.
The deployment comes in the run-up to next autumn's presidential elections in Afghanistan, which the Taliban are expected to try to undermine.
US president-elect Barack Obama has announced that he plans to withdraw US forces from Iraq and increase troop deployment in Afghanistan by about 20,000 soldiers.
More than 60,000 foreign soldiers - including more than 30,000 US troops - are currently based in Afghanistan, partly with the International Security Assistance Force (ISAF) under NATO command, and partly under US-led coalition forces.
Gates is set to keep his post under Obama's new administration.
While security in Iraq has improved in the wake of an increased deployment of US troops, the situation in Afghanistan has deteriorated sharply over the past few years despite the increase in international troops.