The specific assertions about Iran’s role went beyond those made publicly by senior American officials, though Gen. Michael V. Hayden, the director of the Central Intelligence Agency, did tell Congress earlier this month that “the Iranian hand is stoking violence” in Iraq.

The American intelligence on Hizbollah’s role is based on human sources, technical means, and interviews with detainees captured in Iraq, American officials said.

American officials say that the Iranians have also provided direct support to Shi’ite militias in Iraq, including explosives and trigger devices for roadside bombs, and training for several thousand fighters, mostly in Iran. The training is carried out by the Iranian Revolutionary Guard Corps and the Ministry of Intelligence and Security, they say. American officials also suspect that in some cases the Iranians may have provided guidance to the Shi’ite militias to attack specific targets in Iraq.