Iran and six world powers began an expert-level meeting about Tehran's nuclear programme on Thursday, part of efforts to reach an agreement by late July on how to resolve a decade-old dispute that has stirred fears of a Middle East war.
The meeting in Vienna of nuclear and other experts from Iran and the United States, France, Germany, Russia, China and Britain was to prepare for a new round of higher-level negotiations next week, also in the Austrian capital.
A spokesman for European Union foreign policy chief Catherine Ashton - whose office is coordinating contacts with Iran on behalf of the big powers - confirmed that the meeting had started but gave no details. Officials earlier said they were expected to last until Saturday.
The April 8-9 meeting of chief negotiators - including Ashton and Iranian Foreign Minister Mohammad Javad Zarif - will be the third round of talks at that level since February.
The aim is to hammer out a long-term deal by July 20 that would define the permissible scope of Iran's nuclear programme in return for a lifting of sanctions that are severely battering its oil-dependent economy.
Both sides have made clear their political commitment to reach a comprehensive agreement but officials acknowledge that success is far from guaranteed in view of decades of mutual mistrust and big differences over the issues involved.
The powers want Iran to significantly scale back its nuclear activities in order to deny it any capability of quickly diverting them to the production of a nuclear bomb, if it decided to "weaponise" its enrichment of uranium.
Iran says its enrichment programme is a peaceful bid to generate electricity and has ruled out shutting any of its nuclear facilities. It denies having any nuclear bomb designs.
US President Barack Obama, like his predecessors, has said that all options are on the table with regard to Iran's nuclear programme, using diplomatic code for the possibility of military action if diplomacy fails to settle the dispute.
In November, Iran and the six nations agreed an interim accord to curb Tehran's atomic activities in exchange for some easing of sanctions. The six-month deal, which took effect on Jan. 20, was designed to buy time for talks on a long-term deal.
Robert Einhorn, a former senior US State Department official dealing with Iran, said the positions of the parties - especially the United States and Iran - remained far apart.
"Key differences exist on the requirements of an acceptable deal, not just among negotiators at the table but also among key players outside the negotiations," Einhorn said in a new report for the Brookings think-tank in Washington.