The US will likely have to retain a sizeable military force in Iraq even after President George W. Bush has left the White House, a leading London-based defence think-tank said on Tuesday.

On the day the number of US soldiers killed in the conflict reached 2,000, the International Institute for Strategic Studies, which publishes a comprehensive account of military forces around the world, said that plans by the US to shift combating insurgency to Iraq's own army had not yet borne fruit, while rebels were showing considerable resilience.