The top US commander in Afghanistan has said al-Qaeda will not be defeated unless its leader, Osama Bin Laden, is captured or killed. Testifying to the US Congress, Gen Stanley McChrystal said Bin Laden has become an iconic figure. He said President Barack Obama's deployment of 30,000 extra troops to Afghanistan meant success was possible. But he added the mission was undeniably difficult and the next 18 months will be crucial. The general said that killing or capturing Bin Laden will not spell the end of al-Qaeda, but that the movement could not be eradicated while Bin Laden remained at large. The militant leader is believed to be living in the border region between Pakistan and Afghanistan.