Iran has arrested a member of the negotiating team that reached a landmark nuclear deal with world powers on suspicion of spying, a judiciary spokesman said on Sunday.

The suspect was released on bail after a few days in jail but is still under investigation, Gholamhossein Mohseni Ejei said at a weekly news conference, calling the unidentified individual a "spy who had infiltrated the nuclear team," state media reported.

The deal that President Hassan Rouhani struck last year has given Iran relief from most international sanctions in return for curbing its nuclear programme, but it is opposed by hardliners who see it as a capitulation to the US.

Ejei was responding to a question about an Iranian lawmaker's assertion last week that a member of the negotiation team who had dual nationality had been arrested on espionage charges.

Tehran's prosecutor general on August 16 announced the arrest of a dual national he said was linked to British intelligence, but made no mention of the person being in the nuclear negotiations team. On Sunday, Ejei did not explicitly confirm that the arrested person had a second nationality.

Britain said on August 16 that it was trying to find out more about the arrest of a joint-national. -Reuters