The Iranian-backed Lebanese militant group Hezbollah is providing training in Iran for Iraqi militants, according to interrogation reports cited by US officials, the New York Times reported.
The officials said the information came from four members of Shiite militias who were captured last year in Iraq and questioned separately, according to the newspaper.
The information was given to Iraq's government before an Iraqi delegation went to Tehran last week to present evidence of Iran's backing of Shiite militias in Iraq, the Times said.
It added that it was unclear if the Iraqis had in fact put the evidence forward to Iran.
An Iraqi government spokesman said on Sunday after the delegation's return that Iranian officials had denied any interference in Iraq.
Washington accuses Iran of funding, arming and training Shiite militias to attack US-led troops and Iraqi government forces, despite its public commitment to stabilizing Iraq.
Tehran blames the violence on the presence of US forces.
The Times quoted the officials as saying it appeared that Iran, possibly to be less obtrusive, was bringing small groups of Shiite militants into the country.
There were then taught how to train others back in Iraq in techniques for firing rockets, fighting as snipers and building explosively formed penetrators - a lethal kind of roadside bomb made of Iranian components.
The officials said the training was being conducted at several camps near Tehran overseen by the Quds force of the Islamic Revolutionary Guard Command, with instruction from Hezbollah militants.
The Iraqi government said on Monday that Prime Minister Nuri al-Maliki had ordered the formation of a committee to compile evidence of Iranian interference in Iraq that would then be presented to Tehran.
The Iraqis have said they do not want their territory to become the site of a proxy war between the United States and Iran, which are at odds over Tehran's nuclear ambitions.
