The top US commander in Afghanistan is to be replaced after less than one year in the job. US Defence Secretary Robert Gates says he has asked General David McKiernan to step down to make way for "new thinking" at a pivotal moment in the war. He said that he has recommended Lieutenant General Stanley McChrystal, a former commander of special operations forces in Iraq and Afghanistan, to replace McKiernan. The change comes as President Barack Obama intends to double the size of the US force in Afghanistan to 68,000 by the autumn to combat a rising Taliban insurgency.