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 United States.

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 Aug. 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 Aug. 16 that it was trying to find out more about the arrest of a joint-national.

Iran last month executed a nuclear scientist convicted of spying for the United States, an official said Sunday, acknowledging for the first time that the nation secretly detained and tried a man who was once heralded as a hero.

Shahram Amiri defected to the U.S. at the height of Western efforts to thwart Iran's nuclear program. When he returned in 2010, he was welcomed with flowers by government leaders and even went on the Iranian talk-show circuit. Then he mysteriously disappeared. He was hanged the same week that Tehran executed a group of militants, a year after Iran agreed to a landmark accord to limit uranium enrichment in exchange for the lifting of economic sanctions.

Amiri first vanished in 2009 while on a religious pilgrimage to Muslim holy sites in Saudi Arabia. A year later, he reappeared in a series of contradictory online videos filmed in the U.S. He then walked into the Iranian-interests section at the Pakistani Embassy in Washington and demanded to be sent home.

In interviews, he described being kidnapped and held against his will by Saudi and American spies. U.S. officials said he was to receive millions of dollars for his help in understanding Iran's nuclear program.

Iranian judiciary spokesman Gholamhosein Mohseni Ejehi said Amiri "had access to the country's secret and classified information" and "had been linked to our hostile and No. 1 enemy, America, the Great Satan."

The spokesman told journalists that Amiri had been tried in a death-penalty case that was upheld by an appeals court. He did not explain why authorities never announced the conviction, though he said Amiri had access to lawyers.

News about Amiri, born in 1977, has been scant since his return to Iran. Last year, his father told the BBC's Farsi-language service that his son had been held at a secret site. Ejehi said Amiri's family mistakenly believed he received a 10-year prison sentence.