The group Jaish al Adl claimed reponsibility for the attack, which also wounded ten Guard members, Fars reported.
Wednesday's violence comes during a month in which Iran has been commemorating the 1979 Islamic Revolution which swept the USA -backed Shah from power.
"I think the fact that they are not aiming to issue any agreed text but rather are just attempting to use their own statement on behalf of everybody else shows they don't have any respect for it themselves", he said.
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Iran increasingly has blamed the militant attacks targeting it on Saudi Arabia and the United Arab Emirates.
"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.
The bomber struck as the troops were returning from a patrol mission on the border with Pakistan, where Baluchi separatist and jihadist groups have rear bases, the Guards said.
Ten others on board the bus were wounded, the agency said.
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They declined to say to what extent these disclosures compromised USA intelligence operations targeting Iran. In Iran, she has appeared to work at times to publish anti-American articles for the state-owned Press TV.
Jaish al-Adl claimed Wednesday's bombing in a statement online.
In the case of Jundallah, Pakistan assisted Iran in apprehending its leader, whom Tehran executed in 2010.
Suicide bombings are rare in Iran where security is pretty stable.
And most recently, an attack on a military parade in September in Iran's oil-rich southwest killed over 20 and wounded over 60. But the focus is expected to be on Iran and what Secretary of State Mike Pompeo has referred to as countering its "malign influence". There have been sporadic protests in the country as well, incidents applauded by Trump amid Washington's maximalist approach to Tehran. Mark my words: "Iran WILL bring terrorists & their masters to justice", Iranian foreign minister Mohammad Javad Zarif reacted in social media at the time.
Iran's foreign minister, Mohammad Javad Zarif, described the conference convened by the USA as the Warsaw Circus, and said it was "no coincidence that Iran is hit by terror on the very day" that the talks began.
"The Warsaw conference, I believe, is dead on arrival".