US secretary of state John Kerry on Thursday said United States and other major powers are not in a rush to reach a nuclear deal with Iran, suggesting an accord was unlikely hours ahead of a deadline set by the US Congress for a quick review.
Adding to signs that a deal was not close at hand, a top adviser to Iran's supreme leader was quoted by Iranian news agency Tasnim as saying Tehran's "redlines" should be respected in talks aimed at a deal under which Iran would curb its nuclear programme in exchange for sanctions relief.
"A deal can be reached only if our redlines are respected," said Ali Akbar Velayati, the adviser to Supreme Leader Ayatollah Ali Khamenei. He also accused Kerry of making comments that were "part of America's psychological warfare against Iran."
Velayati's reported remarks came after a senior Iranian official in Vienna accused the United States and others nations of shifting their positions and backtracking on an April 2 interim agreement that was meant to lay the ground for a final deal.
"We're here because we believe we are making real progress," Kerry told reporters in the Austrian capital. "We will not rush and we will not be rushed."
However, Kerry said Washington's patience was not unlimited. We can't wait forever," he said. "If the tough decisions don't get made, we are absolutely prepared to call an end to this."
He did not say how much longer the talks could continue. Shortly after Kerry spoke, the White House said the talks would not likely drag on for "many more weeks."
Briefing reporters on condition of anonymity, the senior Iranian official sought to put the onus on the West for any failure to reach an agreement.
"There have have been changes of position ... particularly since last night," said the official, "Suddenly everyone has their own red lines. Britain has its red line, the US has its red line, France, Germany ..."
The official also said that Iran was looking forward to seeing whether the United States would "abandon its obsession with sanctions."
Negotiators have given themselves until the end of the day on Friday. But if a deal is not reached by 6:00am in Vienna (0400 GMT), the sceptical Republican-led US Congress will have 60 days rather than 30 days to review it, extra time US President Barack Obama's administration worries could derail it.
The central bargain of an interim deal struck on April 2 in Lausanne, Switzerland as well as of the final deal that the two sides are now trying to work out is to limit Iran's nuclear work in return for easing economic sanctions crippling its economy.
Iran's deputy foreign minister Abbas Araqchi has said the main text of a final agreement, as well as five technical annexes, were "around 96% complete."
While the lifting of sanctions was largely agreed, Araqchi said Tehran's demand for an end to a UN Security Council arms embargo was among the most contentious unresolved points.
Other sticking points in the negotiations have included Iran's research and development on advanced centrifuges and access to Iranian military sites and nuclear sites.
Tehran says a UN embargo on conventional arms has nothing to do with the nuclear issues and must be lifted in any deal. Western countries do not want allow Iran to begin importing arms because of its role supporting sides in Middle East conflicts.
Iran has powerful support on this issue from Russia. Foreign minister Sergei Lavrov said at a summit of BRICS countries - Brazil, China, India, Russia and South Africa - that the UN arms embargo should be among the first sanctions lifted.
Over the past two weeks, Iran, the United States, Britain, France, Germany, Russia and China have twice extended deadlines for completing the long-term nuclear agreement.
In a sign that the Friday morning US congressional deadline was likely to be missed, energy secretary Ernest Moniz, a key member of the US negotiating team, was due to fly to Portugal on Friday to accept an honour and make a speech, returning on Friday evening.
Western countries accuse Iran of seeking the capability to build nuclear weapons, while Tehran says its programme is peaceful. A deal would depend on Iran accepting curbs on its nuclear programme in return for the easing of economic sanctions imposed by the United Nations, United States and European Union.
A successful deal could be the biggest milestone in decades towards easing hostility between Iran and the United States, enemies since Iranian revolutionaries stormed the US embassy in Tehran in 1979.
It would also be a political success for both Obama and Iran's pragmatic President Hassan Rouhani, who both face resistance from powerful hardliners at home.