The authorities in Iran say more than 10 people have been arrested in the past year accused of spying on its atomic programme for Israel and the US.
Iran denies claims that it wants to build nuclear weapons
An official said that three of those arrested had been working within the state's nuclear programme itself.
Intelligence Minister Ali Yunesi said those arrested had been working in Iran for the CIA and Mossad.
In August, Tehran announced the arrest of a number of spies accused of supplying information to other states.
"More than 10 nuclear spies were arrested during the current Iranian year," Mr Yunesi is quoted as saying by the official Irna news agency.
"They have been working for the CIA and Mossad and were arrested in Tehran and Hormuzgan" in Southern Iran, he added.
The people are being held in the custody of the revolutionary court, he said.
The US accuses Iran of seeking nuclear weapons.
Iran denies this, saying that its nuclear development programme is purely for peaceful, energy-generating purposes.