Geneva: A top Iranian negotiator voiced optimism on Saturday that talks with the United States seeking to nail down a complex nuclear deal could succeed if Washington showed "good will".
"We remain hopeful and I think that if the other side has the necessary good will and determination, it will be possible to reach a deal," Iran's deputy foreign minister Abbas Araghchi told the Fars news agency.
His comments came on a third day of talks with senior US officials in Geneva aimed at accelerating progress towards a comprehensive deal which would rein in Iran's nuclear programme in exchange for relief from a tight network of global sanctions.
Iranian negotiators were also set to meet with Russian officials later in the day, before full negotiations on Sunday with the so-called P5+1 group -- the United States, Britain, China, France, Russia and Germany.
Following an interim accord in November 2013, two deadlines for a final deal have been missed, and now a third one is looming on July 1.
"The negotiations are continuing in a completely serious atmosphere and both parties want to reach an agreement, but problems, chasms and differences also exist," Araghchi acknowledged.
A Western source close to the talks meanwhile said they did not seem to be moving forward significantly and that the biggest stumbling block was on the Iranian side.
"The Iranians have not yet made enough gestures to enable us to reach a good deal that would ensure a substantial reduction of their residual (uranium) enrichment capacity, so we collectively can be assured they don't have the technical capacity to rapidly develop a nuclear bomb," he said.
The source argued that any deal needed to ensure that Iran's "breakout capacity" for making a nuclear bomb was at least one year to give the international community time enough to act.
If the global powers go along with "a weak deal, that would send a disastrous signal to the rest of the world on the issue of proliferation," he said.
Iran insists it is only interested in nuclear energy and adamantly denies it is pursuing atomic weapons.
Among issues complicating negotiations are hardliners on both sides who appear willing to torpedo the efforts.
The new Republican-controlled US Congress is considering a fresh sanctions bill, despite strong opposition from President Barack Obama, who has threatened to veto any such legislation that lands on his desk.