The chief of Iran's Revolutionary Guard on Monday accused the United States, Britain and Pakistan of having links with the Sunni militants responsible for a homicide bombing that killed five senior Guard commanders and 37 others.
"Behind this scene are the American and British intelligence apparatus and there will have to be retaliatory measures to punish them," Major General Mohammad Ali Jafari said, vowing a "crushing" response.
Iranian President Mahmoud Ahmadinejad said those behind Sunday's bombing are hiding across the border in Pakistan, and in a phone call with his Pakistani counterpart on Monday he demanded their arrest.
"The presence of terrorist elements in Pakistan is not justifiable and the Pakistani government needs to help arrest and punish the criminals as soon as possible," state TV quoted Ahmadinejad as telling Asif Ali Zardari.
Earlier Monday, an Iranian military official went as far as to raise the prospect of a possible military offensive into Pakistan against the group blamed for the attack.
"There is even unanimity that these operations (could) take place in Pakistan territory," the ISNA news agency quoted MP Payman Forouzesh as saying.
The Sunni rebel group known as Jundallah, or Soldiers of God, has waged a low-level insurgency in Iran's southeast to protest what it says is the government's persecution of an ethnic minority there claimed responsibility for Sunday's attack. The claim was posted Monday on an Islamic Web site that usually publishes Al Qaeda statements. Its authenticity could not be verified.
The official IRNA news agency said Sunday the dead included the deputy commander of the Guard's ground force, Gen. Noor Ali Shooshtari, as well as a chief provincial Guard commander for the area, Rajab Ali Mohammadzadeh. The other dead were Guard members or local tribal leaders. More than two dozen others were wounded, state radio reported.
The headquarters of Iran's armed forces blamed the bombing on "terrorists" backed by "the Great Satan America and its ally Britain," Fars News Agency said Sunday.
"Not in the distant future we will take revenge," Iran's statement read, according to Reuters. Iran's forces claim the country "will clear this region from terrorists and criminals."
"The global arrogance, with the provocation of its local mercenaries, targeted the meeting of the Guard with local tribal leaders," said the Guard statement read out on state TV.
The United States, however, condemned the attacks on Sunday and denied any involvement.
"We condemn this act of terrorism and mourn the loss of innocent lives. Reports of alleged U.S. involvement are completely false," U.S. State Department spokesman Ian Kelly said in a brief statement.
The Revoutionary Guard commanders were inside a car on their way to a meeting with local tribal leaders in the Pishin district near Iran's border with Pakistan when an attacker with explosives blew himself up, IRNA said.
Iran's state-owned English language TV channel, Press TV, said there were two simultaneous explosions: one at the meeting and another targeting an additional convoy of Guards on their way to the gathering.
The region's top prosecutor was quoted by the semi-official ISNA news agency as saying the Sunni rebel group Jundallah claimed responsibility for the blast.
There was no immediate statement directly from the group.
The group accuses Iran's Shiite-dominated government of persecution and has carried out attacks against the Revolutionary Guard and Shiite targets in the southeast.
That campaign is one of several ethnic and religious small-scale insurgencies in Iran that have fueled sporadic and sometimes deadly attacks in recent years — though none have amounted to a serious threat to the government.
The Guard commanders targeted Sunday were heading to a meeting with local tribal leaders to promote unity between the Shiite and Sunni Muslim communities.
In April, Iran increased security in Sistan-Baluchistan Province, at the center of the tension, by placing it under the command of the Guard, which took over from local police forces.
The 120,000-strong Revolutionary Guard controls Iran's missile program and has its own ground, naval and air units.
Iran's parliamentary speaker, Ali Larijani, condemned the assassination of the Guard commanders, saying the bombing was aimed at disrupting security in southeastern Iran.
"We express our condolences for their martyrdom. ... The intention of the terrorists was definitely to disrupt security in Sistan-Baluchistan
The Associated Press contributed to this report.