Iraq's rebuilding is being undermined by continuing insurgent attacks and will cost more than the $56bn (£32bn) forecast, US officials say.
Iraq's oil exports have been limited by insurgent attacks
Special Inspector-General Stuart Bowen said the cost of reconstruction would be much higher, but did not suggest what the new figure would be.
Mr Bowen said Iraq's infrastructure was also more dilapidated than expected.
His comments came as a director of the US Government Accountability Office said attacks were not decreasing.
In his testimony to the US Senate Foreign Affairs Committee, Joseph Christoff said newly declassified figures went against previous statements by US and Iraqi officials, who said the insurgency was abating.
"There are peaks and valleys, but if you look at every peak, it's higher than the peak before," he said.
Ebb and flow
The charts included in the GAO's report show the number of attacks in December was almost 250% higher than in March 2004.
The most recent peak was in October, when 3,000 attacks were recorded around the time of the referendum on the new constitution.
The report cited a senior officer in the US military as saying that "attack levels ebb and flow as the various insurgent groups - almost all of which are an intrinsic part of Iraq's population - re-arm and attack again".
Though Mr Christoff found the number of attacks on Iraqi security forces had increased as more troops became operational, the majority of attacks targeted the forces of the US-led coalition.
However, most of the fatalities came from attacks on Iraqi civilians, a small proportion of the total.
Though the continuing insurgency was the primary cause for the increasing cost of Iraq's reconstruction, Mr Bowen told the Senate committee that failing to deliver power and fuel had also been critical.
Less oil and electricity is being produced in Iraq now than before the US-led invasion in 2003.
Mr Bowen said this would affect the cost to the international community because the World Bank and US had expected a large proportion of the $56bn needed between 2003 and 2007 to be funded by oil revenues.
"The irony is that the oil, which was prophesied by many who came before this committee as the way you pay for all of it, doesn't pay for all of it,'' Committee Chairman Richard Lugar said.