President Hassan Rouhani yesterday vowed revenge against the "mercenary group" behind a suicide bombing which killed 27 people in southeastern Iran and accused the US and Israel of supporting "terrorism".
"We will certainly make this mercenary group pay for the blood of our martyrs," the official IRNA news agency quoted the Iranian president as saying in response to Wednesday's attack.
"The main root of terrorism in the region is America and Zionists, and some oil-producing countries in the region also financially support the terrorists," he added.
Rouhani was speaking at Tehran's Mehrabad Airport before leaving for the Russian resort of Sochi for a summit with his Russian and Turkish counterparts Vladimir Putin and Recep Tayyip Erdogan on the future of war-battered Syria.
Wednesday's attack, which targeted a busload of Revolutionary Guards in the volatile southeastern province of Sistan-Baluchistan, was one of the deadliest on Iranian security forces in years.
The bomber struck as the troops were returning from a patrol mission on the border with Pakistan, where Baluchi separatist and jihadist groups have rear bases, the Guards said.
The attack came on the same day as the United States gathered some 60 countries in Poland for a conference on the Middle East and Iran which they hoped would increase pressure on Tehran.
Wednesday's bombing was claimed by the jihadist Jaish al-Adl (Army of Justice), which is blacklisted as a terrorist group by Iran, the SITE Intelligence Group reported.