A suicide bomber killed at least 27 members of Iran’s elite Revolutionary Guard Wednesday in a region where security forces are battling militants from the country’s Sunni Muslim minority, Iranian state media reported.

The al Qaeda-linked Sunni group Jaish al Adl — Army of Justice — which wants greater rights for the ethnic minority Baluchis, claimed responsibility for the attack, which also left 13 Guard members wounded, the semi-official Fars news agency reported.

The bomber at the wheel of a vehicle loaded with explosives attacked a bus transporting Guard members.

A video posted by 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 drug smugglers operate with impunity.

The explosion was so powerful that the bus was turned into a twisted pile of metal.

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 Guard, which reports directly to Supreme Leader Ayatollah Ali Khamenei.

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,” Fars quoted him as saying.