In a quarterly report to Congress, the Defense Department said Iran's "malign influence" was one of a number of factors that still weighs on the security situation in Iraq even though violence has fallen to levels not seen since 2004.
"In the months ahead, Tehran will likely seek to influence Iraq by identifying and supporting pro-Iranian individuals and parties in their bids to win upcoming elections," the report said.
"Iran continues to pose a significant threat to Iraq's long-term stability, territorial integrity, and political independence," it said.
"Despite persistent promises to the contrary, Iranian behavior continues to reflect a fundamental desire to oppose the development of a fully secure and stable Iraq."
The report accused Iran of trying to derail, both in public and covertly, a long-term security agreement between the United States and Iraq.
At the same time, "it continues to host, train, fund, arm and direct militant groups intent on destabilizing Iraq," the report said.
"Countering malign influence and balancing soft Iranian influence remain priorities to stabilize Iraq and ensure the sovereignty of its people," it said.
Geoff Morrell, the Pentagon press secretary, said there was "persistent evidence" that Iran continues to supply so-called Shiite "special groups" with training and weapons, including armor-penetrating explosives.
The report alluded to a rout last year of Iranian-backed groups in southern Iraq, saying Iran has since renewed its focus on developing political and economic clout with Iraq's Shiite parties.
At the same time, it also has sought to strengthen ties with smaller, more effective militant groups, the report said.
"Some militants who had sought refuge in Iran are now attempting to return to Iraq, where they would be capable of resuming attacks on Iraqi and coalition forces," the report said.
The report noted other potential trouble spots despite an otherwise improving security situation.
They included a rise in assassinations of judicial and legislative officials, and violence against Christians in the northern Nineveh province.
In addition ethnic tensions in the northern oil city of Kirkuk and the return of refugees "have the potential to halt or reverse progress," the report said.
But in a sign of the times, the report said Iraqis are now less concerned about the violence than about the quality of daily life.
"The lack of essential services has now replaced security as the most important concern in the minds of most Iraqis, many of whom are not satisfied with the quality or availability of food, clean water, electricity, sewage services, and health care," it said.
