The defense lawyer of two Iranian-American dual nationals sentenced to jail in Iran for "collaboration with a belligerent state," says he is going to demand their "conditional release".

The announcement by the lawyer comes less than a week after Iran's Foreign Minister Mohammad Javad Zarif put forward the idea of a prisoners swap between Iran and the United States, and one day after Tehran's prosecutor allegedly "resigned."

The United nations had called on Iran last year to release the two dual nationals, Baqer and Siamak Namazi, father and son, as their detainment "violates international laws."

The defense lawyer, Mehrdad Qorbani, told reporters that he is going to call for their conditional release as they have already served four years in jail.

He said he has made the decision after Tehran's prosecutor Abbas Jafari Dolatabadi announced he is handing over his office to a new prosecutor.

Qorbani said he did not have much hope in arranging a renewed hearing for the case as Iran's Supreme Court has rejected his appeal. "However, we are going to appeal again as Tehran's prosecutor has been replaced."

He said based on Iranian laws, Siamak Namazi, who has been sentenced to 10 years in jail, is eligible for conditional release as he has already served four years.