As US Secretary of State John Kerry said on Wednesday the issue of whether Iran will ultimately be allowed to enrich uranium will not be decided in an interim deal under discussion between major world powers and Iranian officials in Geneva.
"Whatever a country decides or doesn't decide to do, or is allowed to do under the rules, depends on a negotiation," Kerry told reporters.
"We are at the initial stage of determining whether or not there is a first step that could be taken, and that certainly will not be resolved in any first step, I can assure you," he added.
Meanwhile, the third round of nuclear talks that resumed in Geneva between the Islamic Republic and world powers on Wednesday ended after a brief fist session, AFP reported.
According to diplomats, the crunch negotiations lasted only five to 10 minutes.
"This was just a brief introductory session," AFP quoted an anonymous diplomat as saying."There will now be bilateral meetings."
Iranian Foreign Minister Javad Zarif wrote on his Facebook page on Wednesday that he and EU foreign policy chief Catherine Ashton were due to meet for "serious and detailed" talks on Thursday.
"So far, we have progressed based on our plans and we agreed to start serious and detailed negotiations about the agreement with Lady Ashton tomorrow," Zarif said.
A senior US administration official said earlier Wednesday it would be "very hard", though still possible, to reach an initial nuclear deal with Iran in talks in Geneva this week.
"I think we can [get a deal], whether we will, we will have to see because it is hard. It is very hard," the official told reporters after talks between six world powers and Iran resumed in the Swiss city.
The official also said that the vast majority of sanctions on Iran would remain in place after any preliminary accord on limiting its disputed nuclear program, and that Washington would "vigorously" implement them.
World powers resumed efforts to clinch a preliminary deal to curb Iran's nuclear program kicking off two days of closed-door nuclear talks in Geneva on Wednesday, with Russia and Britain confident that agreement can be reached.
Seeking to end a long standoff and head off the risk of a wider Middle East war, the United States, Russia, China, France, Britain and Germany came close to winning concessions from Iran on its nuclear work in return for some sanctions relief at negotiations earlier this month.
Policymakers from the six have since said that an interim accord on confidence-building steps could finally be within reach, despite warnings from diplomats that differences remain and could still prevent an agreement.
British Foreign Secretary William Hague said the remaining differences are narrow and a historic deal is within reach.
"It is the best chance for a long time to make progress on one of the gravest problems in foreign policy," Hague told a news conference during a visit to Istanbul.
Russian Foreign Minister Sergei Lavrov said earlier: "We hope the efforts that are being made will be crowned with success at the meeting that opens today in Geneva."
Iranian Supreme Leader Ayatollah Ali Khamenei said Tehran would not step back from its nuclear rights and he had set "red lines" for his negotiators in Geneva. But Tehran wanted friendly ties with all countries.
"We want to have friendly relations with all nations and peoples. The Islamic system isn't even hostile to the nation of America, although with regards to Iran and the Islamic system, the American government is arrogant, malicious and vindictive," Khamenei said, according to his official website.
Khamenei also criticized France, which had spoken against the deal as proposed earlier, for "succumbing to the United States" and "kneeling before the Israeli regime". France said the comments were unacceptable.
Prime Minister Binyamin Netanyahu flew to Russia on Wednesday to appeal for tougher terms in any accord with Iran after failing to convince the United States that world powers are pursuing a bad deal.
Israel sees a nuclear-armed Iran as a mortal threat and wants its uranium enrichment capabilities dismantled and its enriched uranium removed.
The last meeting with the six powers stumbled over Iran's insistence that its "right" to enrich uranium be recognized, and disagreement over its work on a heavy-water reactor near Arak, which could yield plutonium for atomic bombs once it becomes operational.
Iranian Foreign Minister Zarif has since indicated a way around the first sticking point, saying Tehran has the right to refine uranium but is not insisting others recognize that right.
A UN report last week showed Iran had stopped expanding its enrichment of uranium and had not added major new components at Arak since August, when moderate Hassan Rouhani replaced hardliner Mahmoud Ahmadinejad as president.
Nuclear analyst Ali Vaez of the International Crisis Group think-tank said the "body language" showed that the sides were ready for a deal, pointing to Iran slowing its nuclear push and Washington refraining, so far, from imposing more sanctions.
"[They] have demonstrated that they are looking to transform stumbling blocks into stepping stones," Vaez said.