Iran said Saturday it has arrested the head of a US-based "terrorist group" accused of being behind a deadly 2008 bombing in the southern city of Shiraz and of other, abortive attacks.

The group's "Jamshid Sharmahd, who was leading armed and sabotage operations inside Iran, is now in the powerful hands" of Iran's security forces, state television said in a report citing a statement from the intelligence ministry.

The statement did not elaborate on where or when the leader of the opposition royalist group known as the Kingdom Assembly of Iran, or Tondar (Farsi for Thunder), was arrested. According to the statement, he had orchestrated the April 12, 2008 bombing in a packed mosque in Shiraz that killed 14 people and wounded 215.

Iran hanged three men convicted of the bombing in 2009, saying they had ties to the monarchist group. It said they had been taking orders from an Iranian US-backed "CIA agent" to try to assassinate a high-ranking official in Iran.

They were 21-year old Mohsen Eslamian and Ali Asghar Pashtar, 20 -- both university students -- as well as Rouzbeh Yahyazadeh, 32. The three were found guilty of being "mohareb" (enemies of God) and "corruption on the earth" by a revolutionary court in Tehran.

Iran hanged two other convicted members of the group in 2010, who had "confessed to obtaining explosives and planning to assassinate officials". The statement issued on Saturday said that Tondar had plotted several other "big operations" which failed.

It said that Tondar had planned to blow up a dam in Shiraz, use "cyanide bombs" at a Tehran book fair, and plant an explosive device at the mausoleum of the Islamic republic's founder, the late Ayatollah Ruhollah Khomeini.

It was not clear how Iran arrested the US-based Sharmahd. Iran's intelligence ministry announced the arrest of a former opposition figure in similarly mysterious circumstances in October last year.

Ruhollah Zam, described as a "counter-revolutionary" by Iranian authorities, was sentenced to death last month over "corruption on earth". Zam, who reportedly lived in exile in Paris, ran a channel on the Telegram messaging application called Amadnews and was accused of sparking unrest during anti-government protests last year.