On Sunday, an anonymous group of senior US officials showed journalists what they said was proof that Iranian agents had smuggled weapons to Iraq, including "explosively formed penetrators", a form of roadside booby-trap.
These bombs, they said, have killed 170 American and allied troops and wounded 620 more since May 2004.
US defence officials refused to allow reporters to name them or record their briefing, but released pictures of reported Iranian arms.
Meanwhile, US Democrats say they are sceptical of their government's claims that Iran is secretly channelling weapons to Iraqi fighters, arguing that the issue is best resolved through negotiations rather than confrontation.
But the disclosures came less than a week after congress released a scathing report by Thomas Gimble, the acting Pentagon inspector general, in which he argued that Douglas Feith, former US undersecretary of defence, had manufactured "inappropriate" intelligence reports linking Saddam Hussein and al-Qaeda to bolster the case for an invasion.
And several Democratic senators said these circumstances were fuelling their suspicions about the Bush administration's real motives.
Christopher Dodd, a Democratic US senator, said: "I look at this with a degree of scepticism, based on the record that these intelligence operations have provided us in the past."
Dodd told CBS television he had no doubt that Iran played a role in the current developments in Iraq, but believed the issue should be resolved through diplomatic initiatives.
John Kerry, former Democratic presidential nominee senator, expressed a similar view, acknowledging that he had no doubt that there were "Iranian instigators, agents in Iraq".
Kerry assured the administration's new "evidence of Iranian meddling will be met by "a sceptical Congress, and appropriately so, because of the last experience with Iraq".
In a separate development, the EU has solved a technical snag involving Spain and Britain that had threatened to hold up implementation by the UN sanctions bloc on Iran, EU president Germany has said.
Frank-Walter Steinmeier, the German foreign minister, made the announcement on Monday before a meeting of EU foreign ministers in Brussels. Germany currently holds the EU presidency.
"Both states concerned presented a solution and this morning this was received without any further discussion," Steinmeier said
The snag centred on Spain's refusal to accept an EU text listing Gibraltar as among "competent authorities" to apply the sanctions, reflecting the centuries-old dispute between Britain and Spain on Gibraltar.
Steinmeier said EU states would look at an Iranian offer to resume talks on its disputed nuclear programme.