In an interview with national radio, the US envoy for talks to revive the nuclear deal with Iran, Robert Mali, said Tehran had added demands that were not related to discussions about its nuclear program.

Mali added: "They added demands that I think anyone looking at would see nothing to do with the nuclear deal, things that you wanted in the past."

Mali said Iran had added new demands, including in last week's most recent negotiations in Doha.

These include what the United States and the Europeans said could not be among the negotiations to reinvigorate the 2015 nuclear agreement with world powers.

"The debate that is needed is a fact now not between us and Iran, although we are ready to do so, but between Iran and itself, it needs to reach a decision on whether it is now ready to return to compliance with the agreement," Mali said.

A senior U.S. official told Reuters last week: "The chances of reviving the 2015 Iran nuclear deal became worse after indirect negotiations between the United States and Iran in Doha ended without progress.

He added: "You can describe the Doha negotiations at best as stalled and at worst as backward, but at this point, stumbling is practically backward."

The official did not elaborate on the Doha talks, in which EU officials acted as mediators between the two sides in an attempt to revive the 2015 nuclear deal, which limited Iran's nuclear programme in exchange for easing economic sanctions.