Iranian revolutionary forces have been infiltrating Iraq, US Defence Secretary Donald Rumsfeld has said.
Policing the border between Iraq and Iran has proved difficult
"They [Iran] are currently putting people into Iraq to do things that are harmful to the future of Iraq," Mr Rumsfeld told a news conference.
"We know it, and it is something that they... will look back on as having been an error in judgement," he added.
His comments come amid an upsurge in sectarian violence in Iraq, touched off by the bombing of a sacred Shia shrine.
Asked whether the alleged insertion of Iranian forces into Iraq was backed by the central government in Tehran, Mr Rumsfeld said: "Of course, the Qods force does not go milling around willy-nilly, one would think."
The Qods Brigades are the counter-intelligence division of Iran's Islamic Revolution Guards Corps (IRGC), a force separate to the regular military that protects Iran's constitution and Islamic system. It operates both inside and outside Iran.
General Peter Pace, chairman of the US military's Joint Chiefs of Staff, said there had also been some improvised explosive devices (IEDs) and weapons that "we believe are traceable back to Iran".
A claim by Britain last October that the technology for these devices used in attacks on British troops in southern Iraq could be traced to "Iranian elements or Hezbollah" sparked a heated dispute.
IEDs employing shaped charges and infra-red triggering have killed at least 10 British soldiers in the south of Iraq since May last year, and were first seen in Lebanon where they are used by Hezbollah, which is supported by Iran and Syria.
Yet Iran has denied that it is in any way responsible for supplying the technology.
Speaking to reporters at the Pentagon, Mr Rumsfeld also rejected suggestions civil war was engulfing Iraq, hitting out at what he said were exaggerated media reports of violence there.
"I do not believe they're in a civil war today," he said.
He added: "There's always been a potential for a civil war. That country was held together through a repressive regime that put hundreds of thousands of human beings into mass graves."
But he acknowledged that the 22 February attack on the mosque in the city of Samarra had delayed efforts to form a unified government in Iraq.