Iranian media had previously reported that a bomb had gone off in a crowded mosque in the southern city of Shiraz on Saturday evening, leaving more than 160 people wounded in addition to the 10 killed.

"Last night's explosion in Shiraz was as a consequence of an accident and not the planting of a bomb," IRNA, the official news agency, quoted the deputy interior minister in charge of national security, Abbas Mohtaj, as saying.

He did not give details, but state Press TV television said the blast may have been "caused by explosives left behind from an earlier exhibition commemorating" the 1980-88 Iran-Iraq war.

Fars News Agency, a semi-official Iranian group, carried a similar report. "The cause of the incident was probably laxness since a defense fair was held at this place some time ago. There is a possibility that the remaining ammunition at this place was the factor behind this explosion," it quoted the commander of the security forces in the southern province of Fars, Ali Moayedi, as saying.

The blast occurred Saturday at 9 p.m. at the Shohada mosque, where the secretary to the city's Friday prayer leader preached every week about the Wahhabi faith, an austere version of Sunni Islam, and Bahaism, a religion the Iranian government condemns, Fars reported.

Bombings are rare in Iran, and the government has punished those involved in such attacks severely. Iranian Sunni militants have been involved in several blasts in the past few years. A Shiite cleric known for his stance against Wahhabism was killed in the southern city of Ahwaz last June.

Late Saturday, Fars quoted a police official as saying that a "handmade" device had been planted in the Shohada mosque.

Iran's Foreign Ministry spokesman said Sunday that the investigation was continuing.

State television urged people in Shiraz, a city of more than one million people and a popular tourist destination, to donate blood for the wounded and said all nurses in the city had been called to report for work.

A 20-year-old woman who was wounded said about 800 people had been inside the mosque.

"After we heard an explosion, there was smoke everywhere," said the woman, Saeedeh Ghorbani.