A suicide car bomb attack which killed 27 members of Iran's elite Revolutionary Guards will lead the Islamic Republic to wage an "unrelenting battle against terrorism," Iranian Vice President Eshaq Jahangiri says.
The attack was carried out in a southeast region of Iran where forces are facing a rise in attacks by militants from the country's Sunni Muslim minority.
The Sunni group Jaish al Adl (Army of Justice), which wants greater rights for ethnic minority Baluchis, reportedly took responsibility for Wednesday's attack.
"This incident will not damage the resolute will of the people in defending the Islamic revolution and the national resolve for an unrelenting battle against terrorism will become more resolute than ever," Jahangiri said, according to the Iranian Students' News Agency (ISNA).
A suicide bomber driving a vehicle laden with explosives attacked a bus transporting members of the Guards, the force said.
A video posted by semi-official agency Fars showed blood and debris at the site of the attack on the road between the cities of Zahedan and Khash, a volatile area near the Pakistan border where militants and smugglers operate.
The explosion was so powerful that the bus was turned into a twisted pile of metal, a photo published on Fars showed.
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.
They are the most powerful force in Iran and report directly to Supreme Leader Ayatollah Ali Khamenei.
Twelve members of the Guards were among 25 people killed last autumn in an attack by gunmen on a military parade in the city of Ahvaz.
The militant Sunni group Islamic State and an Arab separatist group claimed responsibility for that attack.
Foreign ministry spokesman Bahram Qassemi vowed retaliation.
"The self-sacrificing military and intelligence children of the people of Iran will take revenge for the blood of the martyrs of this incident," he reportedly said.
In October, Jaish al-Adl claimed responsibility for kidnapping 10 Iranian security personnel. Some of them have been released.