On Tuesday, 16 people were reported killed after armed militants laid siege for hours to the American Embassy and NATO headquarters in Kabul, the Afghan capital.

The US ambassador to Afghanistan, Ryan Crocker, attributed the attack to the Haqqani network, a group allied with al Qaeda and the Taliban based on the Pakistani side of the Afghan-Pakistan border.

US officials said they believed the Haqqani network also carried out a truck bombing on Saturday at a NATO outpost in Afghanistan's Wardak province that killed four civilians and wounded 77 U.S. troops.

Defence Secretary Leon Panetta warned Pakistan on Wednesday the United States would "do everything we can" to defend US forces from Pakistan-based militants staging attacks in Afghanistan.

Reuters