Iran has warned it will take revenge against the US and Britain, accusing them of being involved in a suicide bombing that killed several Revolutionary Guard commanders.
General Noor Ali Shooshtari was among the Revolutionary Guard victims
Six senior commanders and dozens of civilians died in the attack.
The headquarters of the armed forces blamed the bombing on "terrorists" backed by "the Great Satan America and its ally Britain", the semi-official Fars News Agency reported.
"Not in the distant future we (Iran) will take revenge."
Earlier, Iranian state television said Sunni rebels carried out the bombing.
It said the militant group Jundallah (Soldiers of God) had claimed responsibility for the attack, the deadliest in Iran in recent years.
State media cited sources as saying "Britain was directly involved" - a common tactic used to divert attention away from the country’s internal problems.
Pishin region of Iran
Attack was near border with Pakistan
The UK Foreign Office said it rejected "in the strongest terms" any assertion of British involvement.
"The British Government condemns the terrorist attack in the Province of Sistan and Baluchistan in Iran and the sad loss of life which it caused," it added in a statement.
The US also condemned the bombing and denied it had anything to do with it.
"We condemn this act of terrorism and mourn the loss of innocent lives. Reports of alleged US involvement are completely false," US State Department spokesman Ian Kelly said.
The allegations of foreign involvement are likely to raise tension between Tehran and the West, a day before nuclear talks in Vienna.
The military commanders were inside a car on their way to a meeting with tribal leaders when an attacker with explosives strapped to his body blew himself up.
The victims include General Noor Ali Shooshtari, the deputy commander of the Guard's ground force, and Rajab Ali Mohammadzadeh, the Guard's chief provincial commander, the IRNA news agency said.
Up to 30 other people, including civilians and tribal leaders, are reported to have been killed and more than 20 wounded.
The attack took place in the south-eastern Pishin region near Iran's border with Pakistan.
Iran's parliamentary speaker, Ali Larijani, condemned the assassination, saying the bombing was aimed at disrupting security in the region.
"We express our condolences for their martyrdom," he said.
"The intention of the terrorists was definitely to disrupt security in Sistan-Baluchistan Province."
iranian revolutionary guard
Revolutionary Guard soldiers
Over the past few years Jundallah has waged a low-level insurgency and has been at the centre of several violent attacks.
In May, the group claimed responsibility for a suicide bombing at a Shi'ite mosque that killed 25 people in Zahedan.
Thirteen members of the group were convicted for the attack and hanged in July.
Jundallah accuses the mostly Shi'ite government of persecution.
This latest bombing comes at a time when Iran is still reeling from the fallout of its disputed June presidential elections.
A successful attack on such a high profile target marks a significant victory for the attackers.
The Revolutionary Guard was created after the 1979 Islamic Revolution as an ideological and impenetrable wall to defend Iran's clerical rule.
The 120,000-strong elite force controls Iran's missile program and has its own ground, naval and air units.
Today the unit has evolved into a socio-military force with political and economic influence.
It is now an inextricable part of Iran's power structure.
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