The Iranian Foreign Ministry issued a statement saying the perpetrators behind the attack will not go unpunished.
And in October, Jaish al-Adl kidnapped at least 10 security personnel, including several Revolutionary Guards, at a border post in Mirjaveh. He did not specify which enemies or what actions Tehran would take. Riyadh has denied the charges.
The truck was loaded with explosives when it detonated and struck the bus carrying members of the guard who were coming back from "a border guarding mission", Fars reported.
"They used the leverage they have with various countries in order to attract more people to this conference".
The bus was turned into a twisted pile of metal, a photo published on Fars showed.
Reuters could not independently verify the images.
The militant Sunni Muslim separatist group Jaish al-Adl claimed responsibility for the attack. "The enemies will receive a very firm response from the Revolutionary Guards like before". It runs a business empire in Iran worth billions of dollars.
It came on the same day that a conference convened by the USA in Warsaw was to include discussions on what Washington describes as Iran's malign influence across the wider Middle East.
Rouhani called on Iran's neighbors to assume their "legal responsibilities" and not allow "terrorists" to use their soil to prepare attacks.
The latest attack, one of the deadliest ever carried out against the top military force, comes as Iran is trying to parry what it sees as a US-led war on its economy after Washington re-imposed crippling sanctions against the country. Tehran showed off ballistic missiles in defiance of United States efforts to curb its military power.
Khamenei ordered immediate action against those responsible for the attack.
Iran had enjoyed relative stability compared to Arab neighbors who have grappled with political and economic upheaval touched off by popular uprisings in 2011.
And in early December, two people were killed and around 40 wounded in an attack in the strategic port city of Chabahar, on the province's Arabian Sea coast, which Zarif blamed on "foreign-backed terrorists".