Republicans and Democrats on Tuesday grilled General David Petraeus, the US ­military commander in Iraq, over the “surge” in a sign that Congress is not convinced that the policy has been a success.

Gen Petraeus and Ryan Crocker, the US ambassador to Iraq, met intense scepticism from the Senate armed services and foreign relations committees when they provided a positive picture of a surge that was designed to give the Iraqi government “breathing space” to achieve political reconciliation.