President Bush has insisted a branch of Iran's Revolutionary Guards is linked to some attacks on US troops in Iraq.
The US was "certain", he said, that the force was providing a weapon known as an EFP, which the US says has been used in particularly deadly attacks.
But he said he did not know who was directing the force, and denied laying the groundwork for an attack on Iran.
Mr Bush also defended his strategy for securing Iraq a day after the House of Representatives began debating it.
He admitted that it would take time to establish security in Baghdad, and said that violence would continue, but said it was vital to US security to succeed in Iraq.
"If we fail there, the enemy will follow us here. I firmly believe that," he said.
The US House of Representatives on Tuesday began debate on a resolution opposing the president's plan.
The non-binding resolution is expected to pass easily, with as many as several dozen members of Mr Bush's Republican party joining the Democratic majority.
Mr Bush appeared to be trying to steer a fine line on Iran.
Unnamed US officials in Baghdad said at the weekend that the "highest levels" of the Iranian government were supplying weapons to Shia militants in Iraq.
But top uniformed personnel - including the highest-ranking US military officer, General Peter Pace - have refused to confirm that accusation in recent days.
Mr Bush said Iran's Quds Force - a branch of the Revolutionary Guards charged with exporting the Iranian revolution - was the source of the weapons.
But he said he did not know who was giving them their orders.
"I don't think we know who picked up the phone and said to the Quds Force: 'Go do this.'"
The Revolutionary Guards report to Iran's Supreme Leader, Ayatollah Ali Khamenei.
Mr Bush denied he was attempting to provoke Iran, insisting he was only seeking to protect US troops.
He also appeared to suggest there was no point in talking directly to Iran at the present time.
"If I thought we could achieve success, I would sit down with Iran," he said.
But he insisted Tehran must "have a verifiable suspension" of its alleged nuclear weapons programme before the US would engage in direct talks.