Story Highlights • NEW: Percussion device at school causes no deaths or injuries, official says

• NEW: Gunmen then fire at electric plant, spokesman says

• Security forces reportedly surround house containing gunmen

• 11 killed when Iranian military bus bombed in same city Wednesday

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TEHRAN, Iran (CNN) -- A bomb went off Friday night at a girls school in the southeastern Iranian border city of Zahedan, according to the semi-official FARS News Agency.

No one was hurt or killed in the 10 p.m. (1:30 p.m. ET) blast, a military spokesman told the Web site of Iran's state television, Islamic Republic of Iran Broadcasting.

The spokesman, identified as Col. Goodarzi, told IRIB that based on initial information from the scene, a number of militants exploded a percussion bomb in the city and fired at an electricity plant before running away and hiding at a nearby house.

Security forces have surrounded the building in an attempt to arrest them, he said.

Electricity has been cut, and reporters are barred from the area, but gunfire could be heard in the streets.

Zahedan is near where Iran, Pakistan and Afghanistan meet.

According to FARS, the leader of the Jondollah group, Abdolmalek Rigy, claimed responsibility for the bombing on MKO-TV. The group also claimed responsibility for a car bomb in the same area Wednesday.

MKO stands for the Mojahedin Khalq Organization, an Iranian terrorist group stationed in Iraq.

On Wednesday, a car bomb struck an Iranian military bus in Zahedan, killing at least 11 people and wounding more than 30. (Full story)

A witness told the state-run Iranian news agency IRNA that the car blew up as soon as the bus, which belonged to the Islamic Revolutionary Guards Corps, arrived to pick up military personnel at a barracks.

The small terrorist group Jondollah claimed responsibility for Wednesday's attack, FARS reported. The group has a history of launching attacks at Iranian border posts. IRNA reported that five people, including a failed suicide bomber, were arrested after the attack.

According to FARS, Iranian officials said the explosives used in that attack were manufactured in the United States. CNN could not verify those claims.