They were also suspected of moving fighters from Iraq to Iran for training, it said.
Weapons cache
US soldiers also found a weapons cache which they said included seven Iranian rockets and an Iranian mortar near Mahmudiya, 30km south of Baghdad.
"As long as profitable weaponry can be made and delivered, the killing [in the Middle East] will continue"
Judiann, Bloomington, US
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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.
The conference in Egypt's Sharm el-Sheikh resort brings together Iraq's neighbours and industrialised powers.