The US military commander in Iraq has accused Iran of providing covert support to Shia extremists in Iraq.
US forces have clashed with Shia fighters ever since 2003
Iran equips and trains Shia militia groups, Gen George Casey said, adding that its influence had risen recently.
Although the US has no evidence that Iranians were operating directly in Iraq, Gen Casey said "surrogates" regularly attacked US troops.
He also suggested that some troops were likely to leave Iraq this year, but no final decision has yet been taken.
He noted that troop levels had fallen since late 2005, and said he was "confident" more troops would leave during the rest of 2006.
Gen Casey is working with the US ambassador to Iraq, Zalmay Khalilzad, to draw up a proposal for potential US troop withdrawals.
He spoke as the US Senate rejected two Democrat measures calling for troop withdrawals to begin later this year.
Democrat leaders in the Senate tabled two proposals - one calling for a phased withdrawal starting in 2006, the other for all troops to be pulled out of Iraq by mid-2007.
Republicans criticised the plans, labelling one "cut and run" and the other "cut and jog".
Speaking alongside Defence Secretary Donald Rumsfeld at the Pentagon, Gen Casey said intelligence on Iranian activities now confirmed previously-held suspicions.
The US has often accused Iran of aiding Shia groups in Iraq, but has offered little proof of Tehran's alleged activities.
BBC world affairs correspondent, Paul Reynolds, says both the American and British military in Iraq have claimed for some time that Iran, or factions within the Iranian government, have been supporting Shias politically and militarily.
For example, the British ambassador to Baghdad William Patey accused the Iranian Revolutionary Guard of helping to supply the technology which has been used in bomb attacks against British troops in the south.
"Since January we have seen an upsurge in their support, particularly to the Shia extremist groups," Gen Casey said.
"They are using surrogates to conduct terrorist operations both against us and against the Iraqi people.
"We are quite confident that the Iranians, through the special operations forces, are providing weapons, IED [improvised explosive device] technology and training to Shia extremist groups in Iraq," he said.
Training was probably carried out in Iran and possibly in Lebanon, Gen Casey said.
He suggested that Lebanon's Iranian-backed Hezbollah guerrillas were also likely to be involved in training Iraqi militias.
However, some analysts said Gen Casey was wrong to accuse Iran.
"It has not been Shia militants who have fuelled the insurgency in Iraq, it's been by all accounts Sunni Islamist groups and the Jihadis from the outside have been coming from Sunni countries such as Syria and Saudi Arabia and Jordan," Karim Sadjadpour, of the International Crisis Group in Washington, told the BBC.
"So to think that Iran is among the reasons why the US enterprise in Iraq has been unsuccessful I think is really a scapegoat."
Direct contacts between the US and Iran over the security situation in Iraq have been suspended recently.
Iran accused the US of exploiting the issue amid continuing diplomatic wrangling over Iran's nuclear programme.