The death toll in Wednesday's bombing was initially reported by Fars as 30, but the agency later revised that number.
A separatist group called Jaish al-Adl, or Army of Justice, has claimed responsibility, according to state-run news agency IRNA.
The explosion struck the vehicle on a desert road near Iran's volatile border with Pakistan and Afghanistan, where the group is known to operate. It comes two days after Iran marked the 40th anniversary of the Islamic Revolution.
Foreign Ministry spokesman Bahram Qassemi said that Iran will seek revenge for the suicide attack, according to Fars.
"The self-sacrificing military and intelligence children of the people of Iran will take revenge for the blood of the martyrs of this incident," Qassemi said.
The bombing happened as world leaders gathered in Warsaw, Poland, for a meeting with US Secretary of State Mike Pompeo. The meeting has been billed as a conference to counter Iran's military expansion.
Iran is embroiled in several regional conflicts, including those in Syria and Yemen.
Nearly a year after President Donald Trump pulled the US out of the Iran's landmark nuclear deal, the country's economy has buckled under aggressive US sanctions.
Prices have soared, the currency has plummeted and food and medical shortages have affected households across the country.