The semi-official Fars news agency reported 40 members of the IRGC were on board the bus at the time of the attack.
The terrorist group has claimed in their post on their telegram channel that all the 40 IRGC personnel have been killed in the attack.
While Jaish al Adl and other groups like it are not regarded as a major security threat, the attack is a fresh blow to the image of the Guards, the most powerful force in Iran, which reports directly to Supreme Leader Ayatollah Ali Khamenei.
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The Guard issued a statement confirming the attack.
Iranian border guards have repeatedly come under attacks by terror outfits active on the Pakistani soil.
Iranian Revolutionary Guards corps is a US designated terror organization.
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.
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Jaish al-Adl - known as JA - is a Sunni insurgent group in Iran's southeastern Sistan and Baluchistan province and has been fighting Iranian security forces in the region.
Private news agency Tasnim reported twenty seven members of Iran's elite Islamic Revolutionary Guard Corps (IRGC) had been killed in the suicide attack in southeast Iran on Wednesday, .
Jaish al-Adl claimed Wednesday's bombing in a statement online.
Iran's Foreign Minister Mohammad Javad Zarif directly linked the meeting to the attack. Thirteen people were wounded. "Especially when cohorts of same terrorists cheer it from Warsaw streets & support it with (Twitter) bots?"
Wednesday's attack was the deadliest in Iran since September, when gunmen killed at least 24 people at a military parade in the south-western city of Ahvaz.
"It is another attempt by the United States to pursue an obsession with Iran that is not well-founded", Zarif told a Tehran news conference.
He earlier blasted the two-day conference being co-hosted by Washington in Warsaw as "dead on arrival".