TEHRAN (Reuters) - The head of Iran’s Revolutionary Guards vowed on Monday to “retaliate” against the United States and Britain after accusing them and neighboring Pakistan of backing militants who blew up six Guards commanders.

Iranian media say the Sunni Muslim insurgent group Jundollah (God’s soldiers) has claimed responsibility for Sunday’s bombing in Sistan-Baluchestan province, which killed 42 people in all.

The incident threatened to overshadow talks between Iran and global powers in Vienna on Monday intended to tackle a standoff about Iran’s nuclear ambitions.

Guards commander-in-chief Mohammad Ali Jafari said Iranian security officials had presented documents indicating “direct ties” from Jundollah to U.S., British and, “unfortunately,” Pakistani intelligence organizations, the ISNA news agency said.

“Behind this scene are the American and British intelligence apparatus, and there will have to be retaliatory measures to punish them,” Jafari was quoted as saying.

Jundollah, which has been blamed for many attacks since 2005 in the desert province bordering Pakistan, says it is fighting to end discrimination against Sunni Muslims by Iran’s dominant Shi’ites. Its leader is Abdolmalek Rigi.

Jafari said Rigi and his plans were “undoubtedly under the umbrella and the protection” of U.S., British and Pakistani organizations, though he limited the threat of retaliation to the United States and Britain.

“TRAINED BY U.S. AND BRITAIN”

Iranian television quoted General Mohammad Pakpour, commander of the Guards’ ground forces, whose deputy was killed in the bombing, as saying:

“The base of the terrorists and rebels has not been in Iran. They are trained by America and Britain in some of the neighboring countries.”

The United States, Pakistan and Britain all condemned the bombing, the bloodiest attack in Iran since the 1980-88 war with Iraq, and denied involvement.

“We reject in the strongest terms any assertion that this attack has anything to do with Britain,” said a spokeswoman at Britain’s Foreign Office. “Terrorism is abhorrent wherever it occurs.”

The bombing of a mosque in Zahedan, capital of Sistan-Baluchestan, reportedly also claimed by Jundollah, killed 25 people in May.

The poor and remote province, mostly populated by Sunni Muslims, borders both Pakistan and Afghanistan and has frequently been the scene of clashes between security forces, ethnic Baluch Sunni insurgents and heavily-armed drug smugglers.

The victims of the bombing in the city of Sarbaz included a number of tribal chiefs who were due to hold a meeting with the Guards to promote Shi’ite-Sunni unity.

The incident raised tension between Iran and major powers before talks at the International Atomic Energy Agency.

On the agenda in Vienna was a proposal that Iran send low enriched uranium abroad to be enriched further and then returned to be used in a reactor where Iran produces medical isotopes.

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The meeting of Iranian, Russian, French and U.S. officials started shortly after state-run Iranian television said Iran would not deal directly with France since it had failed to deliver “nuclear materials” in the past.

It was not immediately clear what effect this would have on the talks.

NEW CLAMPDOWN?

Analysts say Iran’s governing hardliners may use the bomb attack as an excuse to further clamp down on moderate opponents of President Mahmoud Ahmadinejad, whose disputed re-election in June sparked huge opposition protests.

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A study by the Norwegian Defense Research Establishment published on Monday said Jundollah’s existence showed that Iran’s control over Sistan-Baluchestan was precarious, adding:

“It also shows the limits to Islamic unity within the Islamic Republic itself. This deals a blow to the credentials of the revolution and the international revolutionary aspects of (the late Ayatollah Ruhollah) Khomeini’s doctrine,” it said.

“The great paradox is that Iran, which has been active in support of different Islamist movements outside her own territory after the revolution, is now faced with serious armed opposition within her own borders.”

The Guards force, whose influence has increased since Ahmadinejad came to power in 2005, played a key role in suppressing the street protests after the election.

Russian President Dmitry Medvedev offered cooperation in fighting terrorism and extremism in a letter to Ahmadinejad.

“We are ready to cooperate with Iran in countering these threats,” he wrote, according to press service.

Ahmadinejad urged Pakistani President Asif Ali Zardari in a telephone call to help find the perpetrators of the attack, Iran’s IRNA news agency reported.

Pakistani Foreign Ministry spokesman Abdul Basit told the Daily Times newspaper: “Pakistan is not involved in terrorist activities ... we are striving to eradicate this menace.”

Pakistan has backed armed Sunni Muslim groups in the past, particularly in Afghanistan.

Relations between Iran and Pakistan have been generally good in recent years and the neighbours are cooperating on plans to build a natural gas pipeline. But Iran has in the past said Jundollah members have been operating out of Pakistan.

Some analysts believe Jundollah has evolved through shifting alliances with parties including the Taliban and Pakistan’s ISI intelligence service, who saw it as a tool to use against Iran.