TEHRAN, Iran — Iranian officials raised the death toll from an explosion in a packed mosque in southern Iran to 12 on Sunday and said more than 200 had been injured in the blast.

Iranian media initially quoted local officials as saying the explosion Saturday in the city of Shiraz was caused by a homemade bomb and could have been religiously motivated.

But officials gave a different account on Sunday, saying it was an accident possibly caused by ammunition leftover from a recent military exhibition in the mosque.

"It was not because of bombing," said Abbas Mohtaj, deputy interior minister in charge of security. The "explosion was due to an accident which is under investigation," he added.

The cause of the explosion could not be independently verified.

The blast ripped through a mosque packed with about 1,000 worshippers as a cleric was delivering his weekly speech against the fundamentalist Wahabi strain of Islam and the outlawed Bahai faith, the semiofficial Fars news agency said.

The blast was rare in Shiraz, a normally peaceful city about 440 miles south of Tehran. The city is a major draw for foreign tourists who come to see the ruins of nearby Persepolis, the capital of ancient Persia.

Iran is predominantly Shiite Persian but has faced several deadly attacks in recent years said to have ethnic and religious motivation — though none have seriously threatened the government.

In February 2007, a car loaded with explosives detonated by a bus with members of Iran's Revolutionary Guard, killing 11 of them and wounding more than 30 in southeastern Iran. A Sunni militant group seen linked to al-Qaida, Jundallah or God's Brigade, claimed responsibility.

Besides the violence in the southeast, ethnic Arab Sunni militants have been blamed for bombings in the western city of Ahvaz near the border of Iraq — including blasts in 2006 that killed nine people.

Iranian foreign ministry spokesman Mohammad Ali Hosseini said early Sunday that no group has claimed responsibility for the explosion. He spoke before the Interior Ministry official said the explosion was an accident.

The police chief of southern Fars Province, Gen. Ali Moayyedi, also said the explosion was not a bombing and rejected links to "any sort of insurgency."

Moayyedi, in comments carried by state IRNA news agency, said the initial investigation found remnants of ammunition from a military exhibition that was held recently at the mosque.

A witness to the blast, Mostafa Nazari, told The Associated Press that some 1,000 worshippers had gathered at the mosque grounds to hear a cleric speak. He said it was fortunate the blast happened at a part of the building far from the podium, around which most of the audience had crowded.

"Otherwise it would have caused more casualties," Nazari said.

The explosion was so powerful it shook houses a mile away.