He insisted that the July target was indicative of the " increased urgency" with which the administration of Barack Obama, the US president, viewed the situation in Afghanistan.
His comments come as US support for the war in Afghanistan hits an all-time low following the bloodiest month for American soldiers in the country since the Taliban government was toppled in 2001, with 66 servicemen being killed in July.
Obama's surge strategy was premised on sending 30,000 extra troops to Afghanistan for a fixed period of time, with a drawdown beginning in July 2011.
But critics have said that announcing the date has given America's enemies in Afghanistan the impression that they can 'wait out' the surge, compromising its effectiveness.
Administration officials have appeared to backpedal from the date in recent weeks, repeatedly pointing that it will represent only the beginning of the drawdown and not the mass withdrawal of American combat troops from the country.
Hamid Karzai, the Afghan president, has said the Afghan army and police will be able to take responsibility for the country's security by the end of 2014.