A suicide bombing hit a bus carrying members of the Iranian military's elite Revolutionary Guards in the southeastern Iranian province of Sistan Baluchistan, killing 30 people and wounding 10, according to the semi-official Fars News Agency.
The Revolutionary Guards branch in south-eastern Iran said in a statement that a unit of its ground forces had been returning from the Pakistan border area on Wednesday when a vehicle filled with explosives blew up beside their bus. Thirteen people were wounded.
"The suicide attack on an Islamic Revolutionary Guards Corps personnel bus happened on the Khash-Zahedan road", IRNA said. The explosion was so powerful that the bus was turned into a twisted pile of metal, a photo published on Fars showed.
Reuters could not independently verify the images.
Earlier this month, Jaish al-Adl was blamed for an attack on a paramilitary base in Nik Shahr that left one Revolutionary Guard dead and five others wounded.
The province, which lies on a major opium trafficking route, has seen occasional clashes between Iranian forces and Baluch separatists, as well as drug traffickers. The force also operates outside of Iran's borders in countries such as Iraq and Syria and runs a vast business empire.
Iranian foreign ministry spokesperson Bahram Qassemi vowed retaliation for Wednesday's attack.
The attack comes as Iranian officials have said they blame Saudi Arabia and the United States for stirring up dissent in the country.
But economic hardships fueled by US sanctions have triggered waves of protests that sometimes call for Iran's clerical leaders to step down.
Twelve members of the Guards were among 25 people killed last autumn by gunmen on a military parade in the city of Ahvaz in the southwest.
Jaish al-Adl group claimed responsibility for the roadside bomb attack, which came on the day of a US-led conference in Warsaw that included discussions on what America describes as Iran's malign influence across the wider Mideast.