Iranian Sunni rebels said on Sunday they had kidnapped a nuclear scientist and would publish secrets he knows about Iran's nuclear programme unless 200 political prisoners were released.
Authorities dismissed the threat and denied the man was a nuclear expert, saying he had been a labourer for Iran's Atomic Energy Organisation.
Jundollah rebels seized the man they said was an employee of Iran's Atomic Energy Organisation and are holding him in the mountains in their southeastern heartland in Sistan-Baluchestan province, which borders Pakistan and Afghanistan.
"If the regime does not free (the prisoners) by next week all the information extracted from Mr Amir Hossein Shirani will be released and the whole world will become aware of the regime's clandestine nuclear activities," Jundollah said on its website.
Hamid-Khadem Qaemi, spokesman for the Atomic Energy Organisation, said Shirani had worked for a time as a welder and driver for a company linked to the organisation.
Iran says its atomic programme is purely peaceful, but suspicions that it is seeking nuclear weapons have led to the imposition of several rounds of international sanctions by the United Nations, the European Union and the United States.
Jundollah (God's Soldiers) says it is fighting for the rights of the Baluch people facing "genocide". It carried out a double suicide attack that killed at least 28 people on July 15 in revenge for the execution of its leader Abdolmalek Rigi.
Tehran says the group has links to al Qaeda.
Several armed groups hostile to the establishment are active in Iran, including Kurdish separatists in the northwest and some Arabs in the southwest.