Efforts to carve out a landmark nuclear deal have dragged into a 14th day with still no end in sight, as Iran accused the West of back-tracking and Washington said it was prepared to walk away.
A Friday morning deadline to present the deal to the US Congress was missed, doubling the time for American lawmakers to review the accord - if it can be reached - to 60 days and risking delaying key measures such as the lifting of sanctions.
"We're making progress, it's painfully slow... there are still some issues that have to be resolved," said British Foreign Secretary Philip Hammond, after meeting with other ministers.
But Hammond voiced hope that over the next 12 hours experts working behind the scenes "will clear some more of the text and then we can re-group tomorrow to see if we can get over the last hurdles."
Global powers - Britain, China, France, Germany, Russia and the United States -- are still search for an accord to end a 13-year standoff and nail down a deal to put nuclear arms out of Iran's reach in return for lifting biting international sanctions.
'Defending our national position'
Two years of intense negotiations since the election of Iranian President Hassan Rouhani have now come down to a crunch round of talks in Vienna.
"If the West gives up its excessive demands, we will certainly have a good deal in the nuclear negotiations with the P5+1," Es-Hagh Jahanguiri, Iran's first vice President, said.
The Iranian delegation had been "defending our national positions, the government's red lines and our nuclear rights," Jahanguiri said.
Iranian Foreign Minister Mohammad Javad Zarif, appearing on the balcony of the Palais Coburg where the negotiations are being held, appeared to be digging in for more days of talks, insisting: "We're making progress."
|Iranian Foreign Minister Mohammad Javad Zarif, right, insists that negotiations were moving towards a deal [AP]
Asked if he would stay through the weekend, he replied: "It looks like it."
The talks have been bogged down over some of the thorniest issues such as a mechanism for lifting interlinked EU, US and UN sanctions, and ways to dismantle Iran's nuclear capability and verify its claims that it has never sought an atomic bomb.
US Secretary of State John Kerry on Thursday warned the US would not be rushed into a deal, but insisted he would not stay at the negotiating table forever.
"If the tough decisions don't get made, we are absolutely prepared to call an end to this process," Kerry told reporters.
On Friday, as he met again with the other ministers of the so-called P5+1 group without Iran, the top US diplomat insisted: "We're working hard. We're pushing."
Each of the nations in the group "have different positions which makes the task even harder," Zarif told the Iranian TV channel Al-Alam.
The current effort to alleviate international concerns about Iran's nuclear programme - first revealed by dissidents in 2002 - led to an interim deal in 2013 under which Tehran froze parts of its nuclear programme in exchange for minor sanctions relief.
Two deadlines last year - in July and November -- to turn this into a final accord were missed, but in April in Lausanne, Switzerland the parties managed to agree on the main outlines of a deal.