Col. Gregory Julian said the new forces would include four combat brigades with a total of 20,000 soldiers, CNN reported. Another 10,000 would be support troops.
The United States now has 31,000 soldiers in Afghanistan.
Gen. David McKiernan, the U.S. commander in the Asian country, said in October that 20,000 combat troops were needed because of the resurgent Taliban.
Defense Secretary Robert Gates signed an order for the deployment of 3,000 members of a combat aviation brigade to Afghanistan within a year.
Julian said the United States would probably need to keep about 61,000 soldiers in Afghanistan for three or four years, using a "clear and hold" strategy to oust the Taliban.