The most senior US general in Iraq has said he has evidence that Iran was behind Sunday's bombardment of Baghdad's heavily fortified Green Zone.
Gen David Petraeus told the BBC he thought Tehran had trained, equipped and funded insurgents who fired the barrage of mortars and rockets.
He said Iran was adding what he described as "lethal accelerants" to a very combustible mix.
There has as yet been no response from Iran to the accusations.
In response to the news that 4,000 US military personnel have now been killed in Iraq, he said it showed how much the mission had cost but added that Americans were realistic about it.
He also said a great deal of progress had been made because of the "flipping" of communities - the decision by Sunni tribes to turn against al-Qaeda militants.
The extent of this had surprised even the US military, he said.
In an interview with BBC world affairs editor John Simpson, Gen Petraeus said violence in Iraq was being perpetuated by Iran's Quds Force, a branch of the Revolutionary Guards.
"The rockets that were launched at the Green Zone yesterday, for example... were Iranian-provided, Iranian-made rockets," he said, adding that the groups that fired them were funded and trained by the Quds Force.
The attacks led to 15 civilian deaths
"All of this in complete violation of promises made by President Ahmadinejad and the other most senior Iranian leaders to their Iraqi counterparts."
The barrage hit the Green Zone on Sunday morning. Some rockets missed their targets killing 15 Iraqi civilians.
Later in the day four US soldiers died when their patrol vehicle was blown up by a bomb in southern Baghdad, putting the total number of US fatalities above 4,000.
This and other bloodshed on Sunday came despite an overall reduction in violence since last June, when the US deployed an extra 30,000 troops for the surge.
Days earlier, Mr Bush marked the fifth anniversary of the invasion, saying that it had made the world a better place.