The attack came on the heels of Iran's celebrating the 40th anniversary of its revolution, which saw the overthrow of the USA -backed shah as well as the founding of the Revolutionary Guard Corps, which is tasked with preserving the Islamic Republic.
The attack happened in Iran's port city of Chabahar which has been emerging as economic hub between the regional countries.
Jundallah, a Sunni extremist group whose members have joined Jaish al-Adl, claimed responsibility for that attack.
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. Some of its fighters are seasonal, including some from Pakistan, who are called upon for occasional operations against Iranian security forces.
In July at least 10 members of the Revolutionary Guards were killed when insurgents attacked one of their bases along the border with Iraq.More news: USA meeting on Middle East brings together Israel, Gulf Arab states
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On Twitter he asked: "Is it no coincidence that Iran is hit by terror on the very day that #WarsawCircus begins?"
Local tribes control large swaths of Iran's Sistan and Baluchistan regions and neither Iranian nor Pakistani security forces have a permanent presence there.
The attack comes as Iranian officials have said they blame Saudi Arabia and the United States for stirring up dissent in the country.
"I believe it's dead on arrival or dead before arrival", he said at a news conference before the bombing.
Back in October 2018, at least 29 people were killed and 70 others wounded in an attack on a military parade in Iran's southwestern city of Ahvaz. Mark my words: "Iran WILL bring terrorists & their masters to justice", Iranian foreign minister Mohammad Javad Zarif reacted in social media at the time.
The group also said it had carried out two bombings that wounded three police officers in Zahedan at the end of January.