However, US military chiefs said they had no actual evidence of direct attacks.
"Since January, we have seen an upsurge in their support, particularly to the Shia extremist groups," the top US commander in Iraq, General George Casey, told reporters at the Pentagon. "It's decidedly unhelpful."
"We are quite confident that the Iranians, through their covert special operations forces, are providing weapons, IED technology and training to Shia extremist groups in Iraq, the training being conducted in Iran and in some cases probably in Lebanon through their surrogates," he said.
"They are using surrogates to conduct terrorist operations in Iraq both against us and against the Iraqi people," he said at a joint press conference with Defence Secretary Donald Rumsfeld.
Improvised explosive devices (IEDs) like road-side bombs are among the most commonly used weapons in attacks on US forces in Iraq.
"They are providing the materiel to Shia extremist groups that operate as their surrogates," he said.
"There are some indications that Lebanese Hezbollah is also used in some of the training functions for the Iranians."
No evidence
Mr Casey said he had no evidence of Iranians actually inside Iraq directing attacks against Iraqis or US forces, but said he assumed elements in Tehran were guiding the Iranian special operations forces' efforts.
"You would assume that they're not doing that independently, that there is some central direction from somebody in Tehran," the general said.
Asked which Iraqi groups are getting Iranian aid, Mr Casey replied: "We think they're supporting all - not all of the groups, but a wide variety of groups across southern Iraq."
The White House and Pentagon have repeatedly accused elements from Iran's security apparatus of arming Iraqi insurgent groups.
But they concede they have no clear proof that the Iranian government is sponsoring the activities.
Iran is thought to have great influence with Shiite radical cleric Moqtada Sadr and his Madhi Army militia.
Earlier this month, White House national security adviser Stephen Hadley told CNN television that Iran was no longer interested in direct talks with the United States about the situation in Iraq.
"The Iranians have indicated they no longer have any interest in doing that," Mr Hadley said, when asked in an interview with CNN if the US ambassador in Iraq, Zalmay Khalilzad, will be meeting with Iranian diplomats in Baghdad.
However, Iran had indicated in April that it was interested in such talks.
US Secretary of State Condoleezza Rice authorized Mr Khalilzad to reach out to the Iranians, and up to late May US officials were saying that talks could go forward - even though Iran's hardline President Mahmoud Ahmadinejad by then had said the talks would be of no use.