By Louis Charbonneau and Parisa Hafezi
VIENNA (Reuters) - The United States and other major powers are not in a rush to reach a nuclear deal with Iran, U.S. Secretary of State John Kerry said on Thursday, suggesting an accord was unlikely hours ahead of a deadline set by the U.S. Congress for a quick review.
"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."
Over the past two weeks, Iran, the United States, Britain, France, Germany, Russia and China have twice extended deadlines for completing a long-term deal under which Tehran would curb nuclear activities for more than a decade in exchange for sanctions relief.
Kerry's announcement echoed earlier remarks by Iranian Foreign Minister Mohammad Javad Zarif, who said Iran and major powers would not be hurried.
"We're working hard, but not rushed to get the job done," he said on his Twitter account. Speaking later, Zarif said "We will stay here as long as it is necessary."
France's Foreign Minister Laurent Fabius said the parties would continue negotiating overnight to try to resolve the "difficult issues" that remained in nuclear talks. He added that "things are ... going in the right direction."
Negotiators have given themselves until the end of the day on Friday. But if a deal is not reached by 6:00 a.m. in Vienna (0400 GMT), the sceptical Republican-led U.S. Congress will have 60 days rather than 30 days to review it, extra time U.S. President Barack Obama's administration worries could derail it.
Russian Deputy Foreign Minister Sergei Ryabkov said it was possible there would be an agreement in the coming hours. A spokesman for Russian President Vladimir Putin said the Russian leader hoped for compromises that would enable a deal soon.
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.
"96 PERCENT COMPLETE"
Western officials sought to strike a balance between pushing Iran to cut a deal yet suggesting they were not in a hurry to deprive the Iranians of leverage that might come from a belief that the major powers wanted an accord more than they did.
"We are very close but if the important ... decisions are not made in the next hours, we won?t (have) an agreement," EU foreign policy chief Federica Mogherini told CNN. "That moment of truth will come extremely soon, next hours I think."
Iran's Fars news agency quoted an unnamed source blaming the United States for the standoff.
"While the Iranian team is showing flexibility, the Americans are refusing to accept Iran's obvious right, particularly on sanctions," Fars quoted the source as saying.
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 U.S. embassy in Tehran in 1979.
It would also be a political success for both Obama and Iran's pragmatic President Hassan Rouhani, who each face scepticism from powerful hardliners at home.
Kerry and Zarif have met almost daily for two weeks to overcome the last remaining obstacles to a deal. French Foreign Minister Laurent Fabius and his British and German counterparts have also rejoined the negotiations.
The White House said Obama and his national security team held a video conference on Wednesday with Kerry and the U.S. negotiating team in Vienna.
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 U.N. Security Council arms embargo was among the most contentious unresolved points.
Other sticking points in the negotiations have included research and development on advanced centrifuges and access to Iranian military sites and nuclear sites.
Tehran says a U.N. embargo on conventional weapons has nothing to do with the nuclear issues at stake and must be lifted in any 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 at a summit of BRICS countries - Brazil, China, India, Russia and South Africa - that the U.N. arms embargo should be among the first sanctions lifted.
Iran's President Rouhani was also at the summit and met with Putin.
(Additional reporting by John Irish and Arshad Mohammed and Shadia Nasralla in Vienna and Katya Golubkova and Denis Pinchuk in Russia, writing by Louis Charbonneau; Editing by Anna Willard, Peter Graff and Giles Elgood)
