Zalmay Khalilzad said on Tuesday in a joint media interview with General George Casey from Baghdad that success in Iraq was still possible and could be achieved on a "realistic timetable".
Khalilzad said Iraqi leaders had agreed to a timeline to reduce violence, and that a "national compact" should be in place in the next 12 months if the Iraqi government lives up to agreements.
He said the US will succeed in Iraq, but it will take patience and resolve, and the US should redouble efforts to succeed.
He identified Iran and Syria as backers of extremist forces that are spilling blood and preventing the stabilisation of the country.
For his part, Casey, the leading US commander in Iraq, said Iran and Syria have been "decidedly unhelpful" over the US's attempts to stabilise Iraq.
He said Iraqi forces should be able to take full control of security in the next 12 to 18 months with minimal US support.
Casey said the Iraqi army lost 300 men during the past four weeks alone. Talking about al-Qaeda in Iraq, he said the network was wounded but remains lethal.