Iran's Revolutionary Guards today vowed to take revenge after blaming Britain and the US for a suicide bombing that killed six of its commanders and 37 others in one of the country's most unstable provinces.

The attack, which killed the deputy commander of the guard's ground forces, General Noor Ali Shooshtari, and Rajab Ali Mohammadzadeh, the provincial commander for Sistan-Baluchistan, inflicted Iran's worst military casualties in years and raised questions of intelligence and security failures in a region long blighted by a violent Sunni insurgency.

A Sunni group, Jundallah ("soldiers of God"), claimed responsibility and said it was a response to "the constant crime of the regime in Baluchistan". It named the bomber as Abdol Vahed Mohammadi Saravani.

Iranian media said the attacker had detonated a bomb belt as Revolutionary Guard commanders arrived for a meeting with tribal elders in a sports hall in Pishin, near Iran's frontier with Pakistan. It was the latest in a series of gatherings meant to foster unity in Sistan-Baluchistan, Iran's poorest province, after a spate of attacks.

Those caught in the explosion had to be taken to hospitals more than 150 miles away because Pishin lacked proper medical facilities. Some are understood to have died en route.

The Revolutionary Guards condemned the bombing as the work of "terrorists" supported by "the great Satan America and its ally Britain", and promised to respond.

"Not in the distant future we will take revenge … and Baluchis will clear this region from terrorists and criminals," read a statement released to the semi-official Fars news agency.

The statement echoed another call for revenge by the Iranian president, Mahmoud Ahmadinejad, a former guard. "The criminals will soon get the response for their anti-human crimes," the official news agency IRNA quoted him as saying.

State television cited an "informed source" as saying that Britain was to blame "by organising, supplying equipment and employing professional terrorists".

A US state department spokesman, Ian Kelly, dismissed allegations of American involvement as "completely false", adding: "We condemn this act of terrorism and mourn the loss of innocent lives."

Over the last five years it has become a standard Iranian position that the US-British alliance is a source of unrest in Sistan-Baluchistan and other provinces. Officials point to the presence of Nato forces in neighbouring Afghanistan as a launchpad for Anglo-American interference.

While Iran has blamed Britain and the US for previous attacks on its territory, the latest allegation came as negotiations were due to resume in Vienna over its nuclear programme, which western governments fear may be designed to build an atomic bomb.

Iranian officials have previously linked Jundallah with al-Qaida, although other sources have suggested the group may have connections with the Pakistani Taliban. In Tehran, the Iranian foreign minister summoned the Pakistani charge d'affaires to complain.

The attack appeared to be a direct challenge to the Revolutionary Guards, who took over direct responsibility for Sistan-Baluchistan's security last April. The guards have taken an increasingly prominent role in Iranian affairs in recent times under the auspices of the supreme leader, Ayatollah Ali Khamenei.

Jundallah has taken up arms on behalf of Sistan-Baluchistan's Sunni Baluch population, which it says suffers discrimination at the hands of Iran's Shia rulers. Commanded by Abdolmalek Rigi, the group claims to have killed more than 400 Iranian troops during its insurgency.

It claimed responsibility for a suicide bombing that killed 25 people at a Shia mosque in Zahedan, Sistan-Baluchistan's provincial capital, last May. The authorities responded by hanging 13 group members they said had been involved.

Sistan-Baluchistan lies on a major drug transit route from Afghanistan. Nearly 4,000 Iranian security officers are believed to have been killed in clashes with smugglers since 1979.