They had planned to start drafting a final agreement

But in Vienna, Iran and six world powers have made little progress in talks this week on ending their dispute over Tehran’s nuclear programme.

Iran’s deputy foreign minister Abbas Araqchi told reporters that the meeting had nonetheless been constructive.

“Negotiations will continue,” he said.

“It doesn’t mean we are disappointed. It doesn’t mean that we have failed. No, we will continue our negotiations. This was the first round of real negotiations. In previous rounds of negotiations, we had only brainstorming sessions. The goal of this round was to draft an agreement. We feel that differences are still there and we should wait for the time when we can start drafting, when the positions are closer together.”

This fourth round of negotiations in the Austrian capital lasted three days.

With the 2013 interim deal due to expire in July 2014, a lasting accord is being sought to curb Iran’s contested nuclear activity in exchange for a phased end to sanctions.

“We believe there needs to be some additional realism,” a senior US official said on condition of anonymity, declining to provide details on what issues had caused the most difficulty.

“Time is not unlimited here.”

The US official said the Vienna talks would resume at an unspecified date in June.