An Iraqi police commander and a Western security official in Baghdad have named two powerful Shia militias as top suspects in the abduction of three Americans last weekend in a southern neighbourhood of the Iraqi capital.

They say one of the two militias — Asaib Ahl al-Haq and Saraya al-Salam — were likely behind the attack.

The police commander says "nobody can do anything in that neighbourhood without the approval of those militias." The Western security official confirmed that intelligence assessments had narrowed down the suspects to those the two groups.

Both officials spoke on condition of anonymity because they are not authorized to talk to reporters.

The U.S. Embassy in Baghdad has confirmed that several Americans are missing and says they are working with Iraqi authorities to locate them.