The United States has said it could to send up to 30,000 additional troops to Afghanistan, effectively doubling the number of US forces already there to fight the Taliban.

Before beginning his tour, Holbrooke told a security conference in the German city of Munich that he had "never seen anything remotely resembling the mess we have inherited [in Afghanistan] ... it's going to be much tougher than Iraq".

Holbrooke plans to hold senior-level talks in Pakistan, Afghanistan and India before reporting back to Obama and Hillary Clinton, the US secretary of state.