He appeared to change tack from a weekend briefing in Baghdad by three senior US military officials who alleged that shipments into Iraq of new weapons had been approved at the highest levels in Tehran.
No immediate talks
But he said he could "say with certainty that the Quds force, a part of the Iranian government, has provided these sophisticated IEDs that have harmed our troops," referring to improvised explosive devices like roadside bombs.
"My point is what's worse, them ordering it and it happening or them not ordering it and it's happening?" he said.
Bush also said he would be willing to enter into talks with Iran were it to give "definitive" proof of a suspension to its nuclear activities, but ruled out direct talks for now.
|"I do not know whether or not the Quds force was ordered from the top echelons of government"|
"If I thought we could achieve success, I would sit down [with the Iranians]. But I don't think we can achieve success right now," he said.
Bush's comments came just after Iran's former president, Hashemi Rafsanjani, was quoted as saying that Tehran would "remove obstacles" blocking negotiations with the US if Washington were to show "good will".
"Any time the United States sends a signal showing good will in its dealings with Iran, we will in return remove obstacles in the way of negotiations," the official Islamic Republic News Agency quoted Rafsanjani as saying.
Question of time
On the subject of Iraq Bush said it would "take time" to improve the security situation in Iraq, on the same day Nouri al-Maliki, the Iraqi president, announced a new security clampdown in Baghdad.
Bush said the US must back the Iraqi government's efforts to secure peace.
"The fundamental question is, can we help this government have the security force level necessary to make sure that the ethnic cleansing that was taking place in certain neighborhoods is stopped?" he said.
But, he also said, "the operation to secure Baghdad is going to take time, and there will be violence."
Bush defended his decision to send extra troops to Iraq and said that he respected the opinions of some in Congress who oppose the decision, but criticised a proposed congressional resolution denouncing the military buildup that was being discussed in the house as the president spoke.
"Later this week, the House of Representatives will vote on a resolution that opposes our new plan in Iraq before it has a chance to work," Bush said. "People are prejudging the outcome of this."
On the subject of the nuclear deal reached with North Korea in Beijing, Bush said: "There is a lot of work to be done to make sure that the commitments made in this agreement become a reality."
"But I believe it's an important step in the right direction."
Under the accord, North Korea will be given 50,000 tonnes of fuel aid for closing their key Yongbyon nuclear facility north of Pyongyang and allowing UN nuclear inspectors back into the country.
The US, for its part, would begin the process of delisting the North as a sponsor of terrorism and normalising relations with a country with which it is still technically at war.