The top US military commander in Iraq, Gen David Petraeus, has accused Iran's ambassador of belonging to an elite unit of Iran's Revolutionary Guards.
Gen Petraeus said Hassan Kazemi-Qomi was a member of the Quds Force, which the US believes backs foreign Islamic revolutionary movements.
Gen Petraeus said he had no doubt Iran was behind attacks that had led to the deaths of US soldiers.
Iran has so far not commented, but regularly denies any such involvement.
Separately, the US military said it had captured three Iranian-linked Shia militiamen believed responsible for abducting five British security contractors in May.
Gen Petraeus made his comments during a briefing to journalists at a US military base near Iraq's border with Iran.
Gen Petraeus said the Iranian ambassador to Iraq was a "Quds Force member", but added: "Now he has diplomatic immunity and therefore he is obviously not subject [to scrutiny]. He is acting as a diplomat."
Mr Kazemi-Qomi has twice met US counterpart Ryan Crocker this year to discuss Iraqi stability.
Iran admits the existence of the Quds Force but gives few details of its activities. Analysts believe it is behind funding of such groups as Hamas and Hezbollah.
Gen Petraeus said: "There should be no question about the malign, lethal involvement and activities of the Quds Force in this country."
He said Iran was "responsible for providing the weapons, the training, the funding and in some cases the direction for operations that have indeed killed US soldiers".
Gen Petraeus said Iran was implicated in the car-bomb assassinations of two provincial governors in southern Iraq in August.
The BBC's Jon Brain in Baghdad says some analysts believe the US is deliberately ramping up the rhetoric against the Iranian authorities to prepare public opinion for possible military strikes against Revolutionary Guard facilities within Iran.
Gen Petraeus also delivered a more upbeat message on security in Baghdad in the wake of this year's "surge" by US and Iraqi forces.
He said in some parts of the capital it was secure enough for him to walk down the street unprotected.
"Certainly in places you could do that. You could walk right down Haifa street right now," Gen Petraeus said.
But he added: "Nobody will let me do it."
The security surge that started in February has added about 30,000 extra US troops.
The general said he was not "naive" and knew there was an ever-present threat of bomb attacks.
"If you say: 'Will there be a time when you can walk around Baghdad?', obviously I hope that will be realised in the future."
Meanwhile, the US military said it had captured the three alleged Shia militia fighters in a raid on Saturday in Sadr City, Baghdad.
The fighters were believed to be involved in kidnapping four British security guards and a computer expert from Iraq's finance ministry.
Sunday also saw Iraqi security officials report that bomb attacks in Baghdad had killed at least nine people.
Two of the blasts targeted security patrols, but, in both cases, the victims were civilians.
A third bomb exploded near the Baghdad provincial council building, killing three bystanders.
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In: Iraq, Iran, Middle East
Tags: Iraq, iran, military, infiltrated, hezzbollah, quds, terrorism, terrorists, insurgents, militias, alqaeda, ISI, Coaltion, Iraqi army, Iraqi IP, freedom, suicide bombers, war
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