Iran's Atomic Energy Organisation said Sunday an email server of its subsidiary was hacked in a "foreign" attack aimed at drawing "attention" amid protests over the death of Mahsa Amini.

The Islamic Republic has been gripped by weeks-long demonstrations sparked by the death of 22-year-old Amini on September 16 after her arrest for allegedly violating the country's strict dress code for women.

The street violence has led to dozens of deaths, mostly among protesters but also among the security forces, and hundreds of demonstrators have been arrested.

A group called Black Reward on Friday issued an ultimatum on Twitter, threatening to release documents on Tehran's nuclear programme unless all "political prisoners, prisoners of conscience and people arrested in the recent protests" were released within 24 hours.

Material on social media said to be released by the group on Saturday included a short clip from a purported nuclear site in Iran, as well as documents containing agreements, maps and payslips.

The nuclear agency acknowledged in a statement that a hack had targeted its subsidiary, the Atomic Energy Production and Development Company, but downplayed the importance of the documents.

"Unauthorised access by a source originating from a specific foreign country to the email system of this company led to the publication of the content of some emails on social media," it said in a statement.

These emails contain "technical messages and normal and daily exchanges", it added.

"The purpose of such illegal efforts, which are made out of desperation, is to attract public attention, creating media atmospheres and psychological operations," the statement continued.

Iran in 2015 reached a landmark deal with world powers, after years of negotiations over its nuclear programme.

The agreement, known formally as the Joint Comprehensive Plan of Action (JCPOA), gave Iran sanctions relief in return for restricting its nuclear programme.

It was derailed in 2018 when then-president Donald Trump unilaterally withdrew the United States from it, but on-off talks have taken place since April 2021 in an effort to revive it.