TEHRAN, Nov 6 (Reuters) - Iran's Revolutionary Guards accused Pakistan on Friday of releasing the leader of a Sunni rebel group ahead of a deadly attack in the country's southeastern province, state television reported.

The group Jundollah claimed responsibility for the Oct. 18 attack in Iran's impoverished Sistan-Baluchestan province that killed more than 40 Iranians, including 15 members of the Guards.

"Abdolmalek Rigi was arrested on September 26 in Pakistan's Baluchistan province. But he was released after an hour with the intervention of the Pakistani intelligence service," television quoted Brigadier General Hossein Salami, deputy head of the Guards, as saying.

Iran, a predominantly Shi'ite Muslim country with Sunni minorities, has accused Pakistan, Britain and the United States of backing the Sunni rebel group. London, Washington and Islamabad all deny involvement in the attack last month.

"How can this person move freely unless he is under the protection of the intelligence services?" said Salami.

Iran says those behind the bombing were based in Pakistan and has demanded Islamabad hand Rigi over.

Jundollah (God's soldiers), which accuses the Iranian government of discrimination against Sunnis, has been blamed for many deadly incidents over the last few years.

The group reportedly claimed the bombing of a mosque in Sistan-Baluchestan in May that killed 25 people.

Many minority Sunnis live in the desert area, which has seen an increase in bombings and clashes between security forces, ethnic Baluch Sunni insurgents and drug traffickers.

Iran rejects allegations by Western rights groups that it discriminates against ethnic and religious minorities.

(Writing by Parisa Hafezi, Editing by Elizabeth Fullerton)

(Tehran newsroom, +98 21 8820 8770)

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