US Secretary of State John Kerry says that major world powers will not be "rushed" into a nuclear deal with Iran, adding he is prepared to walk away if "tough decisions" are not made soon.
Speaking on the eve of a deadline to present a deal to the US Congress, Kerry told reporters that because "the stakes are very, very high, we will not rush and we will not be rushed.
"We're here because we believe we're making real progress toward a comprehensive deal. But as I have said many times, and as I discussed with President [Barack] Obama last night, we are not going to sit at the negotiating table forever," Kerry said.
"If the tough decisions don't get made, we are absolutely prepared to call an end to this process."
Earlier, Iran and six world powers meeting in Vienna were said to be close to an historic nuclear agreement that could resolve a dispute that lasted more than 12 years over Tehran's nuclear ambitions.
Russian Deputy Foreign Minister Sergei Ryabkov said on Thursday that he could not rule out the possibility of an agreement in the coming hours, even if there was still doubts over some of the finer details of the deal and if it could be resolved by the end of the day.
Iran's Deputy Foreign Minister Abbas Araqchi said the main text of the agreement, as well as five technical annexes, were "around 96 percent 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.
Over the past two weeks, Iran, the United States, Britain, France, Germany, Russia and China have twice extended a deadline for completing a long-term deal under which Tehran would curb sensitive nuclear activities for more than a decade in exchange for sanctions relief.
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.
It would also be a political success for both US President Barack Obama and Iran's President Hassan Rouhani.
The current round of talks has blown through two deadlines already and has been extended until Friday, but the Obama administration must submit an agreement to Congress before Thursday turns to Friday in Washington if it wants to avoid an extended legislative review.
If the administration misses that target, the congressional review period will double from 30 to 60 days, possibly delaying the sanctions relief that the US would have to give to Iran under the terms of an agreement.
Tehran says the UN embargo on conventional weapons must be lifted in a nuclear deal. Western countries are keen not to allow Iran to begin importing arms because of its role supporting sides in conflicts in the Middle East.
Iran has powerful support on this issue from Russia. Foreign Minister Sergei Lavrov said the UN arms embargo should be among the first sanctions lifted in a deal.
"We are calling for lifting the embargo as soon as possible and we will support the choices that Iran's negotiators make," he said at a summit of BRICS countries - Brazil, China, India, Russia and South Africa.