The younger brother of Iranian President Hassan Rouhani has been detained on financial crime charges, the judiciary has announced.

Hossein Fereydoun is accused of receiving zero-interest loans and influencing the appointment of a bank director who Rouhani had to dismiss for taking an enormous salary. Conservatives have demanded Fereydoun be put on trial.

The detention of Fereydoun on Sunday comes a year after the head of the General Inspection Organisation, Naser Seraj, accused him of financial violations.

"Multiple investigations have been conducted regarding this person. Also, other people have been investigated, some of whom are in jail," deputy judiciary chief Gholamhossein Mohseni Ejeie said in a televised press conference.

"Yesterday, bail was issued for him, but because he failed to secure it he was referred to prison," Ejeie said, adding that he would be released if he secures bail, pending trial.

Fereydoun has acted as a key adviser and gatekeeper to the reformist Iranian leader who was re-elected in May.

He was also involved in the negotiations between Iran and Western powers to reach a nuclear deal, a deal that enraged the country's conservative faction, which controls the judiciary.

He had also served as Iran's ambassador to Malaysia and as a member of the Iranian delegation to the United Nations in New York.