The IRGC personnel were traveling between the cities of Zahedan and Khash, in Sistan and Baluchestan province on Wednesday, when their bus was targeted in a suicide auto bomb attack.
Fars said the attack was claimed by Jaish al-Adl, a jihadist group formed in 2012 as a successor to the Sunni extremist group Jundallah (Soldiers of God), which waged a deadly insurgency against Iranian targets over the past decade.
The incident took place in Iran's Sistan and Baluchistan province, which lies on a major opium trafficking route and has seen occasional clashes between Iranian forces and Baluch separatists, as well as drug traffickers.
The explosion struck the vehicle on a desert road near Iran's volatile border with Pakistan and Afghanistan.
Jaish al-Adl emerged as the main armed opposition movement in the southeast after members of another Sunni group, Jundullah, joined it after their leader was captured on a plane in 2010.
Local tribes control large swaths of Iran's Sistan and Baluchistan regions and neither Iranian nor Pakistani security forces have a permanent presence there.
"Foreign-backed terrorists kill & wound innocents in Chabahar".
The attack comes as Iranian officials have said they blame Saudi Arabia and the United States for stirring up dissent in the country. Prince Mohammed is due in Pakistan, a major recepient of the kingdom's largess, in the coming days.
No further details about the attack were immediately available. Trump withdrew the USA from the accord last May.
Zarif tweeted Wednesday night: "Is it no coincidence that Iran is hit by terror on the very day that #WarsawCircus begins?"
Iranian Foreign Minister Mohammad Javad Zarif predicted the Warsaw summit would not be productive for the U.S. Zarif wrote on Twitter.
He earlier blasted the two-day conference being co-hosted by Washington in Warsaw as "dead on arrival".