CIA denies claims it has been backing Jundullah, the Sunni separatist group which has claimed responsibility for the bombs

Iran is vowing to hunt down a Sunni separatist group which claimed responsibility for a double suicide bombing that killed 28 people at a mosque in the south-eastern city of Zahedan.

Jundullah – Arabic for "the soldiers of God" – said it carried out the twin attacks yesterday at Zahedan's grand mosque in retaliation for the execution of the group's captured leader. Provincial officials said a further 167 people were injured, some of them critically. Three days of mourning were declared. General Hossein Salami, deputy commander of Iran's Revolutionary Guards, claimed in Tehran today that the victims "were martyred by the hands of mercenaries of the US and UK". Ali Mohammad Azad, governor of Sistan-Baluchestan province, blamed "the intelligence services of arrogant powers."

The US and Britain – which are at odds with Iran over its controversial nuclear programme – issued statements condemning the attacks.

Shia worshippers were celebrating the birthday of the prophet Muhammad's grandson Hussein when the first bomb detonated, according to reports from the scene. A second explosion took place 15 minutes later as people rushed to help – a technique used by Sunni groups in Iraq to maximise casualties. The dead reportedly included several Revolutionary Guards.

Iranian media said the aim was to sow discord between Shias and Sunnis in the Sunni majority area, which borders on Pakistan. Zahedan, the capital of Sistan-Baluchestan, has seen several mass casualty attacks in recent years.

In a message sent to al-Arabiyya TV, Jundullah said its intention was to avenge the hanging of Abdulmalik Rigi, the group's leader, in June, after he was captured in Pakistan and extradited to Iran. Rigi's brother Abdelhamid was also executed. Jundullah's website showed pictures of two suicide bombers. They were identified as Mohammad and Mujahid Rigi, apparently members of the leader's clan.

Press-TV, Iran's English-language state broadcaster, alleged that "hardline Wahabis and Salafis trained by the CIA in Pakistan" were behind the bombings. Iran has claimed Rigi confessed that the US had assured him of unlimited aid and funding for the waging an insurgency against the Islamic republic. The US is reported to have a programme of covert operations inside Iran but the CIA has denied backing Jundullah. But Alaeddin Borujerdi, chairman of Iran's parliamentary commission on national security, immediately pointed the finger at the intelligence services of the US, Israel and other western countries. "Such terror operations will not deter Iranian's resolve in fighting against arrogant powers," he told the Irna news agency. Iran was "the main victim" of US-sponsored terrorism, said Heshmatollah Falahatpisheh, another MP.

Official commentators suggested an effort to divert attention from the case of the Iranian scientist Sharhram Amiri, allegedly abducted by the CIA in its quest for the secrets of Iran's nuclear programme but who was allowed to return home on Thursday.

Hillary Clinton, the US secretary of state, condemned the explosions and expressed condolences to relatives of the victims. "This attack, along with the recent attacks in Uganda, Pakistan, Afghanistan, Iraq, and Algeria, underscores the global community's need to work together to combat terrorist organisations that threaten the lives of innocent civilians all around the world," she said.

Alistair Burt, the foreign office minister for the Middle East, also condemned the atrocity. "In targeting a busy mosque, the bombers claimed the lives of peaceful worshippers and passers by," he said. "Our thoughts are with the families of those who have lost their lives so far, and the many more injured."