Robert Malley, the US special envoy to Iran and chief nuclear negotiator, said that his country is unlikely to reach an agreement with Iran to save the 2015 nuclear agreement unless Tehran releases 4 American citizens.

Mali - in an interview with Reuters published on Sunday - repeated the US position in separating the issue of the four detainees from the issue of nuclear activities, but this time came close to saying that their release is a prerequisite for reaching a nuclear agreement, according to the agency.

"They are (two issues) separate and we are following both of them. But I would say that it is very difficult for us to imagine going back to the nuclear deal at a time when Iran is holding four innocent Americans," the US official said.

Speaking from Vienna, where negotiations are underway to revive the nuclear deal, Mali added, "Even while we are in indirect talks with Iran on the nuclear file, we are again in indirect talks with them to secure the release of our hostages."

Rights groups accuse Iran of arresting a number of dual nationals and foreigners for using negotiating cards. Western powers are calling on Tehran to release its citizens, whom it says are political prisoners, but Iran denies holding people for political reasons.