A suicide attack struck a bus carrying members of Iran's Revolutionary Guard, killing as many as 30 personnel of the elite corps.
The bus was travelling on a highway near the town of Chanali in Iran's southeastern Sistan-Baluchistan province, the state-run Fars news agency reported citing officials it didn't name.
Ten others on board the bus were wounded, the agency said. It described the passengers as "shift personnel," but didn't provide any details of their rank.
There was no immediate word on who might have carried out the attack.
In September last year, Isis militants and an Arab group claimed responsibility for a deadly assault on a military parade, also in the country's south, that killed and wounded dozens of people, including members of the Guards.
The latest attack, one of the deadliest ever carried out against the top military force, comes as Iran is trying to parry what it sees as a US-led war on its economy after Washington re-imposed crippling sanctions against the country.
The Islamic Republic is also engaged in a proxy confrontation with Saudi Arabia and Israel across regional conflicts from Syria to Yemen.
A meeting sponsored by the US and held in Poland this week aims in part to build support for American efforts to thwart Iranian influence in the Middle East.
While violence and bomb attacks are not uncommon on both Iran's western and eastern frontiers - bordering Iraq and Afghanistan respectively - audacious and organised assaults that directly target the Islamic Republic's security services are still rare.