Ryan Crocker, who departs later this month after two years as the U.S. envoy in Iraq, said talks he had held with Iranian diplomats to discuss Iraq's security had been fruitless.
"There is also what I would call a terrorist element from some Shia extremists and we believe that they are supported still by elements within Iran," Crocker told the Arabic al-Arabiya television station.
"We have seen a lot of evidence: rockets that are fired on us and on the Iraqis that are made in Iran as recently as 2008, explosively formed projectiles that are produced as a result of Iranian training, and both we and the Iraqis have captured militants who later say they were trained in Iran," he said.
"So the evidence is clearly there, I don't think that's in question. The question is what decisions the Iranians are going to make about their future relationship with Iraq."
Iraqi officials, who last year joined Washington in complaining about Iranian support for militants, have lately said that Iranian interference appears to be abating.
Tehran has always denied supporting militants in Iraq.
"We reject these comments by the U.S. ambassador. If the U.S. forces have evidence, they should present them to the Iraqi government and let the Iraqi government be the judge," Amir Arshadi, media attache at the Iranian embassy, told Reuters.
"Iran does not meddle in Iraq's affairs, and this is confirmed by Iraqi officials."
Attacks by Shi'ite militia in Iraq have fallen dramatically since U.S.-backed Iraqi forces cracked down on them in the first half of last year. Nevertheless, Washington has not stepped back from accusing its arch foes in Tehran of meddling.
Crocker led talks with Iranian officials on Iraqi security in 2007, billed then as a landmark in U.S.-Iranian relations, which have been frozen since the Iranian revolution in 1979. He said the talks yielded few results, and blamed Tehran.
"We approached those talks with a positive spirit and an open mind; unfortunately we did not see results coming out of these discussions and we've not held any direct talks with the Iranians since 2007."
(Reporting by Peter Graff and Wisam Mohammed; writing by Peter Graff; Editing by Michael Christie)
