Iran on Monday accused the United States, Britain and Pakistan of involvement in a devastating suicide bombing which killed more than 40 people and struck at the heart of its security apparatus.
The Islamic republic's supreme leader Ayatollah Ali Khamenei pointed a finger of blame at US intelligence, ISNA news agency reported.
"This terrorist crime revealed the evil face of enemies of security and unity who are supported by intelligence organizations of some arrogant governments," he said, using Iran's term for the United States.
Israeli publication YNet News reported that the Iranians vowed to retaliate, adding: "A commentary by the official news agency called on Iranian security forces 'to seriously deal with Pakistan once and for all.'"
Washington has denied any involvement while condemning Sunday's attack and loss of life as an act of terrorism.
An Iranian general accused Pakistan, Britain and the United States of aiding Sunni rebels behind the suicide attack near the Pakistani border that counted seven military commanders among the 42 dead.
President Mahmoud Ahmadinejad, in a telephone conversation with his Pakistani counterpart Asif Ali Zardari, separately charged that those behind the bombing were in Pakistan and needed to be "quickly confronted."
The suicide bomber blew himself up at a meeting of Iran's elite Revolutionary Guards and local tribesmen in Iran's restive Sistan-Baluchestan province, a hotbed of Sunni insurgency.
Tehran said he attack at a gymnasium in the town of Pisheen was claimed by Sunni militant rebel leader Abdolmalek Rigi, whose Jundallah (Soldiers of God) group has for years been waging war against the Shiite rule of Iran.
The head of the Revolutionary Guards Corps, General Mohammad Ali Jafari, said Monday that an Iranian delegation would head to Pakistan to deliver "proof" that Islamabad was supporting Rigi.
"The delegation will ask for him (Rigi) to be handed over," Jafari was quoted as saying by ISNA.
Jafari also charged that Rigi takes orders from the intelligence services of Britain, Pakistan and the United States.
"The group of Rigi has direct contact with the American and British intelligence services and unfortunately the Pakistani intelligence service," Jafari said.
"He is supported by them and without doubt he is acting under their orders and plans."
Jundallah itself, in a statement posted on the Internet, said the operation's aim was to avenge "the wounds of the Baluch people which have been bleeding for years without end."
It named the bomber as Abdul Wahid Muhammadi Sarawani and said Iranian intelligence officials were among the dead.
"During the past year alone, this regime killed hundreds of (Baluch) youths of this province who together died either by firing squad, execution or martyrdom under torture," it said.
"The Baluch people ... are determined to stand against injustice and to obtain their freedom till the last drop of their blood," the group said.
General Mohammad Pakpour, the head of Guards' ground forces and whose deputy was killed in the powerful blast, also said Washington and London were backing those who launched the attack against Iran's prestigious military force.
"The terrorists were trained in the neighbouring country (Pakistan) by the Americans and British," Pakpour said on state television.
Britain, like the United States, denied the allegations that it aided the rebels. "We reject in the strongest terms any assertion that this attack has anything to do with Britain," a Foreign Office spokeswoman said in London.
Ahmadinejad reiterated that the assault was plotted in Pakistan and urged Zardari to confront the Jundallah rebels.
Jundallah has in recent years repeatedly attacked the Guards, the elite military force set up after the 1979 Islamic revolution to protect the Islamic regime from internal and external threats.
A senior judiciary official in the province, Hojatoleslam Ebrahim Hamidi, said that "more than 30 Sunni tribal chiefs" were among the dead and questioned Jundallah's claims to be promoting the Baluchi cause.
UN chief Ban Ki-moon on Monday issued a statement in which he "strongly condemns yesterday's terrorist attacks."
Sistan-Baluchestan's deputy governor, Jalal Sayyah, meanwhile, said Iran has identified the bomber but no arrests have yet been made. "It is likely that those who supported the bombing have fled" to Pakistan, he said.