The state-run IRNA news agency said the attack took place in Sistan and Baluchistan province, which has a large, mainly Sunni ethnic Baluchi community that straddles the border with Pakistan. It described the passengers as "shift personnel", but didn't provide any details of their rank.
Jaish al-Adl (Army of Justice), a Sunni militant group, has claimed responsibility for the attack, according to the Fars news agency.
The attack came on the heels of Iran's celebrating the 40th anniversary of its revolution, which saw the overthrow of the US -backed shah as well as the founding of the Revolutionary Guard Corps, which is tasked with preserving the Islamic Republic.
The Islamic Republic is also engaged in a proxy confrontation with Saudi Arabia and Israel across regional conflicts from Syria to Yemen.
Iran's Foreign Minister Mohammad Javad Zarif directly linked the meeting to the attack Wednesday. It was the deadliest such attack in Iran in almost a decade. 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. Thirteen people were wounded.
And in October, Jaish al-Adl kidnapped at least 10 security personnel, including several Revolutionary Guards, at a border post in Mirjaveh.
Zarif tweeted Wednesday night: "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.
However, a bombing like this inside of Iran likely will draw an immediate reaction from the Guard, a massive paramilitary organization that both controls Iran's ballistic missile program and vast chunks of its economy.
A coordinated June 7, 2017 ISIS group assault targeted the Iranian parliament and the shrine of Ayatollah Ruhollah Khomeini, the leader of Iran's 1979 Islamic Revolution.
Iran's Shi'ite Muslim authorities say militant groups operate from safe havens in Pakistan and have repeatedly called on the neighboring country to crack down on them. Zarif wrote on Twitter.
"I believe it's dead on arrival or dead before arrival", he said at a news conference before the bombing.