Iran vows revenge on 'mercenaries' behind suicide attack

President Hassan Rouhani vowed revenge Thursday 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.

Sistan-Baluchistan is home to a large ethnic Baluchi community, who straddle the border and who, unlike most Iranians, who are Shiite Muslims, are mainly Sunni.

Rouhani called on Iran's neighbours to assume their "legal responsibilities" and not allow "terrorists" to use their soil to prepare attacks.

"If this continues and they cannot stop the terrorists, it is clear -- based on international law -- that we have certain rights and will act upon them in due time," he said, without elaborating.

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.

Iran quickly linked the attack to the Warsaw conference, where supporters of the formerly armed opposition People's Mujahedeen plan a second day of protests on Thursday.

Dubbing the meeting the "WarsawCircus", Foreign Minister Mohammad Javad Zarif said it was "no coincidence that Iran is hit by terror on the very day" that the talks began in the Polish capital.

"Especially when cohorts of same terrorists cheer it from Warsaw streets & support it with twitter bots? US seems to always make the same wrong choices, but expect different results," Zarif wrote on Twitter.

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. The group was formed in 2012 as a successor to Sunni extremist group Jundallah (Soldiers of God), which waged a deadly insurgency against Iranian targets over the previous decade.

Sistan-Baluchistan has been hit by previous deadly attacks in recent months. On January 29, three members of an Iranian bomb squad sent to the scene of an explosion in provincial capital Zahedan were wounded when a second device blew up as they were trying to defuse it.

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".

In October, Jaish al-Adl claimed responsibility for abducting 12 Iranian security personnel near the border, five of whom were later released and flown home after Pakistani intervention.

Zarif visited Islamabad twice in a month for briefings on the progress of the efforts to secure the captured unit's release.