The trial of Washington Post journalist Jason Rezaian, accused of spying on Iran for the United States, resumed behind closed doors in Tehran on Monday, state media announced.

The case against the 39-year-old, a dual American-Iranian citizen who denies guilt but has been in custody for almost a year, comes with Iran locked in nuclear talks with world powers.

As Iranians waited for news of a possible nuclear deal in Vienna, the official IRNA news agency said the third session of Rezaian's trial -- he first faced the spying charges in May -- had started.

The report gave no other details of the proceedings. The case has been shrouded in secrecy since Rezaian was detained on July 22 last year.

During the last session on June 8, the journalist was given his first opportunity to defend himself.

He is accused of collecting confidential information, cooperating with hostile governments and disseminating propaganda against the Iranian regime.

Rezaian's family has said the reporter is being used as a pawn in an internal political power struggle about the ongoing nuclear diplomacy. His lawyer insists there is "no proof" against him.

Tehran does not recognise dual nationality and says the case is a purely Iranian matter for its judiciary.