"The murder of innocent civilians in their place of worship is an intolerable offense, and those who carried it out must be held accountable," US President Barack Obama said in a statement.
US Secretary of State Hillary Clinton denounced the bombing in the “strongest possible terms” and called for those responsible to be held accountable.
The Sunni insurgent group Jundallah earlier claimed responsibility for the attack on the Zahedan Grand Mosque, in which 27 worshippers were killed and about 100 were wounded, including members of the elite Revolutionary Guard.
Jundallah said the attack was to avenge the loss of their leader Abdolmalek Rigi, who was executed by Iranian authorities last month.
But Gen. Hossein Salami, deputy head of the Revolutionary Guard, told people at the Friday prayers in Tehran that the victims “were martyred by hands of mercenaries of the US and UK.”
He was echoed by influential lawmaker Alaeddin Boroujerdi who said “America should be answerable for the terrorist incident in Zahedan.”
Iran accuses the United States and Britain of supporting the Jundallah in a plot to weaken Tehran leadership, a claim both countries deny.
The United States is locked in a stand-off with Iran over its nuclear program, which Tehran insists is for peaceful purposes but Washington and other major powers fear is a cover to build an atomic bomb.
Jundallah has repeatedly succeeded in carrying out deadly strikes on the Guard, the country’s most powerful military force — including an October suicide bombing that killed more than 40 people.
The new attack was a sign that the group is still able to carry out devastating bombings even after Iran hanged its leader. People were attending a religious ceremony when the first blast went off at the entrance of the mosque in the provincial capital Zahedan.
The male bomber was disguised as a woman, local lawmaker Hossein Ali Shahriari told the ISNA news agency.
Inside the mosque, a cleric was reciting from the Qur’an in front of lines of faithful sitting on the floor when the building suddenly shook from the blast and screams were heard from outside, according to footage taken at the time and aired on Iranian state TV.
As people rushed to help, the second explosion detonated 20 minutes later, causing the majority of the deaths and injuries, ISNA reported. The technique is often used by militants in Iraq to maximize casualties.
Members of the Guard were among the worshippers, particularly because the religious ceremony coincided with Iran’s official Revolutionary Guard Day. The deputy interior minister, Ali Abdollahi, told the Fars news agency that several Guard members were among the dead.
Health Minister Marzieh Vahid Dastagerdi told ISNA on Friday that the death toll stood at 27 but could still rise, with another 270 injured, including 11 in serious condition.
Jundallah has been waging an insurgency for years in the remote Sistan-Balochistan province. The groups says it is fighting for the rights of the mainly Sunni ethnic Balochi minority, which it says suffers discrimination at the hands of Iran’s leadership. Iran has accused the group of links to Al-Qaeda, but experts say no evidence of such a link has been found.
Iran executed Jundallah’s leader in June in Zahedan, a month after hanging his brother Abdulhamid Rigi, who had been captured in Pakistan in 2008 and extradited to Iran.
The group named a new leader, Al-Haj Mohammed Dhahir Baloch.
In a statement posted on its website, Jundallah claimed responsibility for Thursday night’s blast, saying they were to avenge Abdulmalik Rigi’s death. It showed pictures of two suicide bombers wearing explosive vests, identified as Mohammad and Mujahid Rigi, apparently members of the leader’s clan, though the site did not specify their relationship to him.
The group said its “sons of the faith... carried out tonight a heroic unprecedented operation at the heart of an assembly of the Guard at Zahedan,” claiming to have killed more than 100.
