"The president was specifically citing the testimony that our lead intelligence official gave before Congress," White House Spokesman Scott McClellan said at a news briefing.

John Negroponte, US director of national intelligence, made a congressional testimony on Iraq-Iran situation in February.

"Tehran has been responsible for at least some of the increasing lethality of anti-coalition attacks by providing with the capability to build improvised explosive devices (IEDs) in Iraq," Bush quoted Negroponte as saying in a speech on Monday.

Instead of tying such activity to Iran's government, Bush said only that US-led forces had "seized IEDs and components that were clearly produced in Iran."

But one day after Bush's speech, US Defense Secretary Donald Rumsfeld said that the United States cannot prove that Iran's government is behind the alleged movement of Iranian Revolutionary Guard forces and bomb-making equipment into Iraq.

"As to equipment, unless you physically see it coming in a government-sponsored vehicle or with government-sponsored troops, you can't know it," Rumsfeld said.

Besides, General Peter Pace, chairman of the Joint Chiefs of Staff, also said that Washington had on proof that Tehran was behind the alleged movement of Revolutionary Guards, weapons, and IEDs into Iraq.