An Afghan policeman stands guard at a U.S. military base following an attack by Taliban fighters in Torkham near Afghan border with Pakistan in Nangarhar province, eastern Afghanistan, on September 2, 2013. Afghan Taliban insurgent group early Monday morning launched a complex attack on a U.S. forces' base in eastern Afghan province of Nangarhar, sources said. (Xinhua)

Photo taken on Sept. 2, 2013 shows the damaged trucks at a U.S. military base following an attack by Taliban fighters in Torkham near Afghan border with Pakistan in Nangarhar province, eastern Afghanistan, on September 2, 2013. Afghan Taliban insurgent group early Monday morning launched a complex attack on a U.S. forces' base in eastern Afghan province of Nangarhar, sources said. (Xinhua)

Afghan policemen arrive at the U.S. military base following an attack by Taliban fighters in Torkham near Afghan border with Pakistan in Nangarhar province, eastern Afghanistan, on September 2, 2013. Afghan Taliban insurgent group early Monday morning launched a complex attack on a U.S. forces' base in eastern Afghan province of Nangarhar, sources said. (Xinhua)

An Afghan policeman stands guard on a military vehicle at a U.S. military base following an attack by Taliban fighters in Torkham near Afghan border with Pakistan in Nangarhar province, eastern Afghanistan, on September 2, 2013. Afghan Taliban insurgent group early Monday morning launched a complex attack on a U.S. forces' base in eastern Afghan province of Nangarhar, sources said. (Xinhua)

Photo taken on Sept. 2, 2013 shows the damaged trucks at a U.S. military base following an attack by Taliban fighters in Torkham near Afghan border with Pakistan in Nangarhar province, eastern Afghanistan, on September 2, 2013. Afghan Taliban insurgent group early Monday morning launched a complex attack on a U.S. forces' base in eastern Afghan province of Nangarhar, sources said. (Xinhua)

Afghan policemen arrive at the U.S. military base following an attack by Taliban fighters in Torkham near Afghan border with Pakistan in Nangarhar province, eastern Afghanistan, on September 2, 2013. Afghan Taliban insurgent group early Monday morning launched a complex attack on a U.S. forces' base in eastern Afghan province of Nangarhar, sources said. (Xinhua/)

Photo taken on Sept. 2, 2013 shows the damaged trucks at a U.S. military base following an attack by Taliban fighters in Torkham near Afghan border with Pakistan in Nangarhar province, eastern Afghanistan, on September 2, 2013. Afghan Taliban insurgent group early Monday morning launched a complex attack on a U.S. forces' base in eastern Afghan province of Nangarhar, sources said. (Xinhua)

A U.S military helicopter flies over a U.S. military base following an attack by Taliban fighters in Torkham near Afghan border with Pakistan in Nangarhar province, eastern Afghanistan, on September 2, 2013. Afghan Taliban insurgent group early Monday morning launched a complex attack on a U.S. forces' base in eastern Afghan province of Nangarhar, sources said. (Xinhua)