They were also suspected of moving fighters from Iraq to Iran for training, it said.
US soldiers also found a weapons cache which they said included seven Iranian rockets and an Iranian mortar near Mahmudiya, 30km south of Baghdad.
US forces in Iraq detained five Iranians in a raid in January and said they were linked to Iranian Revolutionary Guard networks that provided weapons to fighters. Iran said the five were diplomats and has demanded their release.
Iraq has received promises of co-operation on border security over the past three years but armed groups are still able to smuggle recruits and weapons into the country.
Iraqi diplomats are pressing Condoleezza Rice, US secretary of state, to meet her counterpart, Manouchehr Mottaki, the Iranian foreign minister, on the sidelines of a conference on Iraq on Friday.
The conference in Egypt's Sharm el-Sheikh resort brings together Iraq's neighbours and industrialised powers.