File Photo: Foreign Ministers of P5+1 and Iran at the nuclear talks being held in Vienna, Austria on July 6, 2015. (Reuters)
Iran accused Western powers on Thursday of changing positions in nuclear talks, saying the crunch negotiations in Vienna could still go either way.
US Secretary of State John Kerry said meanwhile as a 13th day of talks went late into the night that he was prepared to walk away if "tough decisions" are not made soon.
"We see certain changes in the positions particularly just last night ... on a lot of issues unfortunately," a senior Iranian official said on condition of anonymity.
The official also accused Western countries of "going back" on parts of a framework accord agreed in April, which the current talks are aimed at finalising, and of not being united.
The emerging deal between Iran and the P5+1 group Britain, China, France, Germany, Russia and the United States is aimed at ending a 13-year standoff by curbing Iran's nuclear activities in exchange for sanctions relief.
There is "one red line for the US, one red line for the UK, one red line for France, one red line for Germany," the Iranian official said.
"It could go either way, this can be a small bridge in the negotiations, it can be a major setback," he said.
"I believe that a comprehensive agreement is within reach, it just requires people to abandon unnecessary or illusional objectives and just move forward with what is at hand and what is achievable."
Earlier Kerry said after talks with the foreign ministers of France, Germany and Britain as well as lower level Russian and Chinese officials that he will not be "rushed" into a deal.
If an accord is not presented to the US Congress before early Friday Vienna time, the review period doubles to a potentially more problematic 60 days.
"We're here because we believe we're making real progress", Kerry said.
But he warned "we are not going to sit at the negotiating table forever."
'The Right Direction'French Foreign Minister Laurent Fabius, announcing he would stay in the Austrian capital Thursday night, said meanwhile that "things are going in the right direction".
British Foreign Secretary Philip Hammond also stayed in Vienna, while his German counterpart Frank-Walter Steinmeier was returning to Berlin. Sources said he would be back early today.
"We are very close, but if the important, historical decisions are not made in the next hours we won't have an agreement", EU foreign policy chief Federica Mogherini told CNN.
"On the other side it's clear that for everybody that making the decisions in one week, two weeks, or three weeks from now will not be easier, but more difficult for everybody," she said.
All sides say huge progress has been made in the past days of negotiations the final stage of marathon talks first launched in September 2013 with most of the accord written.
"The text is done. It's already there. It's a matter of yes or no... the parties have the political space to take political decisions," Mogherini said.
Some of the thorniest issues such as a time frame for lifting sanctions and a UN probe into allegations that Iran in the past sought to develop nuclear weapons, also appear close to resolution.
But Iran's demand that a UN arms embargo be lifted has thrown a spanner in the works. Western nations have balked at the idea, with Tehran still accused of fomenting violence in the Middle East.
Russian Foreign Minister Sergei Lavrov, who was Thursday in the Russian city of Ufa at a summit with emerging economies, threw Moscow's weight behind Tehran on the issue.
"We are in favour of lifting the embargo as soon as possible and will support a decision made by Iran's negotiators," Lavrov told reporters.