The US military accused Iranian intelligence services this evening of providing weapons to militants in Iraq and said gunmen were being trained in Iran in the use of lethal roadside bombs.
US military spokesman Maj-Gen William Caldwell showed journalists in Baghdad weapons that he said were made in Iran. They included mortar rounds and rocket-propelled grenades, which he said were found in Baghdad this week.
Iran tried to step up pressure on Baghdad's government to secure the release of five Iranians being held by US forces. It threatened to pull out of an international conference on Iraq next month, an Iranian newspaper reported.
Tehran says the five, detained in a raid in northern Iraq in January, are diplomats, but Washington accuses them of having links to Iranian Revolutionary Guard networks that it says are training Iraqi militants.
The United States accuses Tehran of trying to destabilise Iraq and says Iranian-made weapons are increasingly being used in attacks in Iraq. Iran denies the accusations.
US-led forces have launched an operation in Baghdad to curb sectarian violence that threatens to erupt into civil war. Maj-Gen Caldwell said that for the third month in a row, civilian casualties had declined in Baghdad but that in the same period there had been an increase in casualties across Iraq.
"Iranian intelligence services are active here in Iraq in terms of both providing funding and weapons and ammunition," he said, appearing to refer to help given to Shia militias.
He said some aid from Iranian intelligence services was also being given to Sunni insurgents.
Iranian officials were not immediately available for comment but have dismissed similar charges made by Washington in the past. President Mahmoud Ahmadinejad has said Washington is trying to hide its own failure in Iraq by blaming others.
Washington has hardened its rhetoric over Shia Iran's alleged role in the war in Iraq and tension has been growing between the two arch-foes over Tehran's nuclear ambitions.