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Iranian police and insurgents clashed briefly after a bomb - described as "sound bomb" - went off in southeastern Iran but injured no one, the country's state-run news agency said on Saturday.
The device exploded late Friday near the site where an earlier explosion this week killed 11 members of Iran's elite Revolutionary Guards in Zahedan, the capital of the Sistan-Baluchestan province on the border with Pakistan.
"The explosion left no casualties or injuries and didn't cause damages," Hasan Ali Nouri, governor of Zahedan, was quoted as saying by the official Islamic Republic News Agency.
IRNA said police and insurgents briefly clashed after the explosion at a school in Zahedan. No one was in the school at the time of the blast.
A Sunni Muslim militant group called Jundallah, or God's Brigade, which has been blamed for past attacks on Iranian troops, had claimed responsibility for Wednesday's bombing when a car bomb blew up a bus carrying Revolutionary Guardsmen, killing 11, in Zahedan.
Iran has accused the United States of backing militants to destabilize the country. Tensions between Teheran and Washington are growing over allegations of Iranian involvement in attacks on US troops in Iraq, and over Iran's controversial nuclear activities.
IRNA quoted an unnamed "responsible official" late Friday as saying that one of those arrested on charges of involvement in Wednesday's bombing, identified as Nasrollah Shanbe Zehi, has confessed that the attacks were part of alleged US plans to provoke ethnic and religious violence in Iran.
The confessions by Zehi helped police detain an unspecified number of Jundallah members and confiscate weapons and documents from the group in a raid Thursday in Zahedan, IRNA also said.
A majority of Iran's population are Shiite Muslims but minority Sunnis live in southeastern Iran.
Friday's blast came just hours after the funeral of the 11 Revolutionary Guardsmen in the provincial capital. Iran's state-run television showed footage of Zahedan residents marching in the streets with the coffins of the killed Guardsmen. The crowd chanted, "death to hypocrites," a reference to the insurgents.
The blasts are a sharp flare-up of violence but the remote southeast corner of Iran, near Pakistan and Afghanistan, has long been plagued by lawlessness. The area is a key crossing point for opium from Afghanistan and often sees clashes between police and drug gangs.
Jundallah, believed by some to have links to al-Qaida, has waged a low-level insurgency in the area and is led by Abdulmalak Rigi, a member of Iran's ethnic Baluchi minority, a community that is Sunni Muslim and also can be found in Pakistan and Afghanistan. Rigi has said his group is fighting for the rights of impoverished Sunnis under Iran's Shiite government.
The semiofficial Fars news agency said Rigi appeared on a station run by the opposition People's Mujahedeen group, which is based in Iraq, minutes before Friday's explosion. The group has long sought to overthrow the Iranian government.
Teheran has often raised concerns that Washington could incite members of Iran's many ethnic and religious minorities against the Shiite-led government in Teheran.
Iran has faced several ethnic and religious insurgencies that have staged occasionally deadly attacks in recent years - though none have amounted to a serious threat to the government.
In December, Jundallah claimed responsibility for kidnapping seven Iranian soldiers in the Zahedan region, threatening to kill them unless group members were freed from Iranian prisons. The seven were released a month later, apparently after negotiations through tribal mediators.
In March 2006, gunmen dressed as security forces killed 21 people on a highway outside Zahedan in an attack authorities blamed on "rebels," though Jundallah was never specifically named.