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By Fredrik Dahl and Reza Derakhshi
TEHRAN (Reuters) - A suicide bomber killed six senior Revolutionary Guards commanders, including two of its top officers, and 29 other people on Sunday in one of the boldest attacks against Iran's most powerful military institution.
The attack highlighted deepening instability in a southeastern region of mainly Shi'ite Muslim Iran bordering Pakistan and Afghanistan. Many minority Sunnis live in the impoverished area, which has seen an upsurge in bombings and other violence.
State media said a local rebel Sunni group called Jundollah (God's soldiers) claimed responsibility for the attack, the deadliest on the elite Guards in recent years, which also wounded about 30 people ahead of a meeting with tribal chiefs.
The talks were part of efforts to foster Shi'ite-Sunni unity and the Guards said the attack was aimed at fomenting sectarian strife in Sistan-Baluchestan province, media said. About 10 senior tribal figures were among the dead.
Iranian officials also accused the United States and Britain of involvement, a charge rejected by Washington. Tehran says the United States backs Jundollah to stir trouble in the border area and has also linked the group to Sunni Islamist al Qaeda.
The armed forces' headquarters issued a statement warning of "revenge," the semi-official Fars News Agency reported.
The southeastern province is the scene of frequent clashes between security forces, ethnic Baluch Sunni insurgents and heavily-armed drug traffickers.
Jundollah, which accuses Iran's Shi'ite-led government of discrimination against Sunnis in the remote desert region, has been blamed for many deadly incidents over the last few years.
President Mahmoud Ahmadinejad said those behind the attack would be "seriously dealt with" and called on Pakistan to help catch and hand them over. Iran has in the past said Jundollah members were operating from its neighbour.
"We were informed that some security agents in Pakistan are cooperating with the main elements of this terrorist incident," Fars News Agency quoted him as saying.
"We ask the Pakistani government not to delay any longer in the apprehension of the main elements in this terrorist attack."
The Foreign Ministry summoned a senior Pakistani diplomat in Tehran and said there was evidence the perpetrators came to Iran from Pakistan. "The Pakistani official assured Tehran his country would take all measures to secure its border with Iran," state television said.
The deputy head of the Guards' ground forces, General Nourali Shoushtari, and its commander in Sistan-Baluchestan province, General Rajabali Mohammadzadeh, were among the dead. Shoushtari was also a senior official of the Guards' elite Qods force.
"Rigi's terrorist group has claimed responsibility for the attack," said state television, referring to Abdolmalek Rigi, leader of Jundollah which is linked by some analysts to the Taliban in Pakistan.
Television showed footage of three bodies covered with blood-stained clothing and of wounded people being taken to hospital. Glass shards and other debris were scattered at the scene of the attack.
The United States condemned the bombing.
"We condemn this act of terrorism and mourn the loss of innocent lives. Reports of alleged U.S. involvement are completely false," State Department spokesman Ian Kelly said.
Most people in Sistan-Baluchestan are Sunni Muslims and ethnic Baluchis. Iran rejects charges by Western rights groups that it discriminates against ethnic and religious minorities.
The Revolutionary Guards is an elite force seen as fiercely loyal to Iran's Supreme Leader, Ayatollah Ali Khamenei. Its power and resources have increased in recent years. It handles security in border areas.
The bombing and allegations of foreign involvement risk overshadowing talks between Iranian and Western officials in Vienna on Monday intended to help resolve a standoff with the West over Iran's nuclear ambitions.
"It could damage that dialogue," defence analyst Paul Beaver said. He said a suicide bombing could indicate links to al Qaeda but he dismissed allegations of a U.S. or British role: "I think it is highly unlikely."
The attack is likely to harden the resolve of the clerical and military establishment in confronting the opposition to a disputed election in June which saw Ahmadinejad's re-election.
Opposition leader Mirhossein Mousavi said he would press ahead with efforts to reform the Islamic Republic despite a post-poll crackdown on protests, his website said on Sunday.
The election plunged Iran into its deepest internal crisis since the 1979 Islamic revolution. The opposition says more than 70 people were killed as Revolutionary Guards and Islamic militia put down the demonstrations that erupted after the vote.
Citing a witness, state television said Sunday's attack occurred when senior Revolutionary Guards officers attending a conference in the southeastern city of Sarbaz went to talk to a group of tribespeople making baskets.
English-language Press TV said the suicide bomber was a tribesman who "detonated his explosives strapped to his body."
Among those killed were also the Guards' commanders in the cities of Sarbaz and Iranshahr.
Jundollah, which claimed responsibility for the bombing of a Shi'ite mosque in May that killed 25 people in the same region, says it is fighting for the rights of minority Sunnis in Iran.
Some analysts believe Jundollah has evolved through shifting alliances with parties including the Taliban and Pakistan's ISI intelligence service, who saw the group as a tool against Iran.
In London, Saman Zarifi, Amnesty International Asia-Pacific Director, told Reuters: "We are very concerned that the Iranians will respond by executing Baluchi detainees. That has been their response to previous such incidents, simply taking people out of prison and killing them."
Iran executed 13 alleged Jundollah members in July.
(Additional reporting by Parisa Hafezi in Tehran, Peter Griffiths in London and Arshad Mohammed in Washington; Writing by Samia Nakhoul; Editing by Janet Lawrence)