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.
Meanwhile, the IRGC Qods Base has issued a statement, condemning the attack as a revenge for the failure of the foreign plots in the aftermath of the celebrations of the 40th anniversary of the 1979 Islamic Revolution.
The restive Sistan and Baluchistan, which is home to many ethnic Sunni Baluch, is one of the most underdeveloped and poverty-stricken areas in the country.
The Revolutionary Guard later reported on its website that 27 were killed and 13 wounded. It was the deadliest such attack in Iran in almost a decade.
Iranian state media have blamed Jaish al-Adl or "Army of Justice", an al-Qaida-linked Sunni extremist group for the bombing.
The bomber at the wheel of a vehicle loaded with explosives attacked a bus transporting Guard members. Some of its fighters are seasonal, including some from Pakistan, who are called upon for occasional operations against Iranian security forces.
"Foreign-backed terrorists kill & wound innocents in Chabahar".
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"Is it no coincidence that Iran is hit by terror on the very day that #WarsawCircus begins?"
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.
Suicide bombings are rare in Iran where security is pretty stable. "Especially when cohorts of same terrorists cheer it from Warsaw streets & support it with (Twitter) bots?"
Iran increasingly has blamed the militant attacks targeting it on Saudi Arabia and the United Arab Emirates.
He earlier blasted the two-day conference being co-hosted by Washington in Warsaw as "dead on arrival". JA was also responsible for an attack in January that wounded three police officers in Zahedan, the capital city of Sistan and Baluchistan province.
"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.