The death toll rose to 42 after an apparent suicide bomb attack against Iran's Revolutionary Guards in southeastern Iran, Press TV reported on Monday.
The dead included seven senior provincial commanders.
The attack on Sunday took place in Sistan-Baluchistan province, near Pakistan as the guards were reportedly on their way to a meeting of tribal leaders in Pishin.
State television reported a group named Jundollah (Soldiers of God) had claimed responsibility for the attack.
Authorities speculated the attack was the work of a group involved in drug trafficking along the borders with neighbouring Afghanistan and Pakistan.
Jundollah is thought to be the main criminal group in the region which has been involved in frequent clashes with Iranian guards at the border lines to Afghanistan and Pakistan.
Thirteen Jundollah members were executed last July in the provincial capital Zahedan. Abdol-Hamid Rigi, the brother of the group's leader Abdolmalik Rigi, is on death row.