According to IRGC Qods Base statement, the bus was attacked by a vehicle bomb packed with explosive material on the road between Zahedan and Khash in the southeastern province.
The suicide bomber struck a bus transporting IRGC members on the road between the cities of Zahedan and Khash in Sistan-Baluchistan Province, the IRGC said in a statement carried by state media.
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.
Reacting to the suicide attack in Sistan-Balochistan province, Revolutionary Guards commander Ali Fadavi was quoted as saying by Fars news agency "our response in defence of the Islamic Revolution will not be limited to our borders..."
"The crime will remain as a "dirty stain" in the black record of the main supporters of terrorism in the White House, Tel Aviv and their regional agents", he said, according to the Reuters news agency.
However, in recent months, there's been an uptick in assaults by the Sunni extremist group Jaish al-Adl, or the "Army of Justice".
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Jaish al-Adl operates primarily in Sistan and Baluchistan, but it also operates from bases in the neighboring Baluchistan province in Pakistan where it gets support from local Baluch tribes.
Almost a year after President Donald Trump pulled the United States out of the landmark Iran nuclear deal, the country's economy has buckled under aggressive USA sanctions.
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. Some of its fighters are seasonal, including some from Pakistan, who are called upon for occasional operations against Iranian security forces.
Iran's Foreign Minister Mohammad Javad Zarif directly linked the meeting to the attack Wednesday.
"The Warsaw conference, I believe, is dead on arrival".