Iran was asking for too much for too little in return during its nuclear negotiations with world powers that wound down Saturday in Kazakhstan, a senior US official said.
"It is fair to say that Iran is willing to take very limited steps on its nuclear programme while expecting very significant results in return," the senior US official told reporters on condition of anonymity.
"They put forward some minimal ideas but expected a great return and a quite disproportionate return," the US official said.
The two-day discussion between Iran and the world powers -- comprised of the five permanent UN Security Council members and Germany and known collectively as the P5+1 -- ended in the Kazakh city of Almaty without an agreement or any decision about when the sides will meet again.
The US official said these talks differed from four previous sessions held in the past two years because they were much more substantive and involved give-and-take by both sides.
"The quality of the discussion was different in the sense that we were talking about all the topics of the confidence-building measure," said the official.
"We didn't do our set pieces. It was really a lot of give and take -- quite different from the last times. But you still have to get to the substance. The substance is not there yet."
The official added that the negotiating process will continue in the same or some other format in the future, pointing out that EU foreign policy chief Catherine Ashton would be calling chief Iranian negotiator Saeed Jalili in a matter of days.
"We believe the process can continue -- how, we are not sure the best way," said the official. "At the moment, we don't know" if there will be a new round of talks between the P5+1 and Iran or some other format."
Yet the official expressed confidence that the change in the substance of the discussion meant that certain progress was being made.
"There may not have been a breakthrough. But there was also no breakdown," the senior US official said.