US Ambassador to Iraq Zalmay Khalilzad has said Iran is publicly professing support for Iraq's political process while backing militias and insurgents.
Mr Khalilzad said militia attacks are killing more than the car bombs
Mr Khalilzad said Iran's security services were training, supplying and financing groups such as the Shia Mehdi Army and Sunni Arab Ansar al-Sunna.
Iran has repeatedly denied US charges of interference in Iraqi affairs.
But last week, Tehran offered to hold direct talks on Iraq with the US, their first public dialogue since 1979.
Iran's supreme leader, Ayatollah Ali Khamenei, publicly endorsed the talks on Wednesday, but warned Washington against "bullying" tactics.
He said a meeting with the US would allow Iran to tell Washington it should leave Iraq and allow Iraqis to govern themselves unhindered.
US President George W Bush has also backed the dialogue as a means to tell Iran "what's right or wrong in their activities inside Iraq".
No date has been set for a meeting.
In an interview with the Washington Post, Mr Khalilzad was asked if any discussions had begun.
"There is nothing new on that," he said.
But the ambassador was more forthcoming about the role of Iran's Islamic Revolution Guards Corps (IRGC), a force separate to the regular military that protects Iran's constitution and Islamic system, and the Ministry of Intelligence and Security (MOIS).
"Our judgment is that training and supplying, direct or indirect, takes place, and that there is also provision of financial resources to people, to militias, and that there is presence of people associated with Revolutionary Guard and with MOIS," he said.
Mr Khalilzad said Iran's alleged backing of Shia militias - which Sunni Arabs have accused of operating death squads with the support of the Shia-led interior ministry - was a concern for the US.
"The militias haven't been focused on decisively yet... That will be tough," he said.
"More Iraqis in Baghdad are dying - if you look at the recent period of two, three weeks - from the militia attacks than from the terrorist car bombings."
Contact with Sadr
In particular, Mr Khalilzad said he was concerned by Iran's ties with the Mehdi Army militia of the radical Shia cleric, Moqtada Sadr.
Moqtada Sadr has called for US-led forces to withdraw
Despite supporters of Mr Sadr winning more than 30 seats in December's parliamentary election, Mr Khalilzad said the US had no face-to-face contact with the cleric.
"No, I don't talk to him, because we don't meet with Moqtada Sadr, but I have sent him messages publicly... We engage him whatever way we can," he said.
"I think that our people advise me against it because there is an indictment against him."
In 2004, an Iraqi judge released an arrest warrant for Mr Sadr in connection with the death of a moderate Shia leader, Abdul Majid al-Khoei, in April 2003.
Mr Khalilzad also stressed the need for Iraqi leaders to form a new government as soon as possible in order to quell the sectarian unrest that was sparked by the destruction of a Shia shrine in Samarra in February.
"I am the one who's saying, 'The country is bleeding, you need to move'," he said.