Four UN rights experts on Wednesday called on Iran to "immediately" annul the death sentence against an academic accused of espionage during nuclear talks with world powers, warning his trial had been marred by violations.

Ahmadreza Djalali, an Iranian emergency medicine specialist resident in Sweden, was sentenced to death in the Islamic republic in October for spying for Israel.

The academic was a visiting professor at Belgium's Vrije Universiteit Brussel when he was arrested during a trip to Iran in April 2016.

He was accused of passing information to Israel's Mossad intelligence service during the negotiations that led to Iran's nuclear deal with world powers in 2015.

Djalali has said he is being punished for refusing to spy for Iran while working in Europe.

In a statement, the four independent experts working with the United Nations warned that "the procedure against him was marred by numerous reports of due process and fair trial violations".

The UN expert on the human rights situation in Iran, alongside the experts on arbitrary detention, torture and summary executions, pointed to reports that Djalali had been held incommunicado, denied access to a lawyer, and forced to confess.

"The execution of Dr Djalali must be immediately halted and he must be released and accorded an enforceable right to compensation and other reparations," the experts said.

"No formal charges were brought against him for nearly 10 months and he was effectively prevented from exercising his right to challenge the legality of detention," they said.

They also maintained that "Djalali's rights to a fair trial before an independent and impartial tribunal and to an effective defence have been violated".

- Forced confession -

During his detention, Djalali was reportedly threatened with torture and with being sentenced to death through an expedited process, according to the statement.

"Following this he was allegedly forced to confess to certain crimes, and forced to repeat dictated statements before video cameras," it said.

Djalali was sentenced to death on October 21, and the experts lamented that, according to information they had received, "he was not given a meaningful opportunity to appeal against his conviction and no defence submission was made".

Tehran's prosecutor general Abbas Jafari Dolatabadi said at the time that he had divulged information that had led to the assassination of two Iranian nuclear scientists in 2010, at the height of tensions over the country's atomic programme.

But Djalali's lawyers have said the evidence in his initial trial was gathered under duress and produced no evidence to substantiate the allegations.

According to Amnesty International, Iran's Supreme Court upheld his sentence earlier this month, prompting the organisation to accuse Tehran of running "roughshod over the rule of law".

The UN experts voiced particular concern over a video prepared by Iran's intelligence ministry that aired late Sunday, in which Djalali confessed to working with a foreign intelligence agency while studying in Europe.

"The timing of the video release, as well the allegations of coercion beg serious questions about its legitimacy," they said.