Eighteen people were killed on Wednesday when a bomb exploded next to a bus in the southeastern city of Zahedan, the official IRNA news agency reported.
The bomb was hidden in a car by "armed rebels and attackers" and exploded as the bus, belonging to the ground forces of Iran's Revolutionary Guards, passed by, the agency reported.
"In this act 18 Zahedan citizens have been martyred. Rebels and those who create insecurity martyred these people in a terrorist act by laying a trap close to a bus," Qassim Rezai, a military commander, was quoted as saying.
It was not clear if those killed were members of the Guards.
Zahedan is close to the border with Pakistan and Afghanistan. There are often clashes between security forces and armed groups in the area involved in the drugs trade.