VIENNA (AFP) - World powers struggled on Saturday to break a deadlock in marathon negotiations with Iran, as they seek a deal curbing Teheran's nuclear ambitions in return for sanctions relief.
On the 15th day of tortuous talks, French Foreign Minister Laurent Fabius warned: "Everything is on the table. It's now time to decide."
A new deadline for a deal is looming on Monday, and all are asking how much longer busy ministers can clear their schedules to hunker down in Vienna amid a myriad of pressing global issues.
An Iranian official told AFP the talks, now entering their third week, could stretch on and on.
"We have no time limit in order to reach a good deal," the senior Iranian official said, while Iranian Foreign Minister Mohammad Javad Zarif was photographed smiling and working on his sunny balcony at the luxury Palais Coburg where the talks are being held.
Iran and the so-called P5+1 group - Britain, China, France, Germany, Russia and the United States - are seeking to curtail Tehran's ability to develop nuclear weapons in exchange for relief from painful sanctions.
But after three previous deadlines in this round of talks - June 30, July 7 and July 9 - were missed, Iran and the US traded accusations in a public spat.
Kerry had Friday offered a glimmer of hope that some progress was being made.
But almost immediately after the top US diplomat emerged from almost 90 minutes of fresh talks Saturday with Zarif and EU foreign policy chief Federica Mogherini, he tweeted that things remained tough.
"Met with @FedericaMog and @JZarif this AM. Still have difficult issues to resolve," he said on Twitter.
Under the parameters of a framework deal reached in Lausanne in April, Iran is due to slash the number of its centrifuges from more than 19,000 to just over 6,000 and cut its stocks of enriched uranium, which can be used to make a bomb, from more than seven tonnes to about 350 kilos.
The aim is to ensure that it would take Iran at least a year - from an estimated two to three months currently - to acquire enough fissile material to build a bomb.
But for weeks experts have been wrangling exactly over how to implement the Lausanne guidelines.
The negotiations have also stumbled over demands that UN nuclear inspectors have access to military sites, amid suspicions Iran sought to develop nuclear weapons in the past, allegations Teheran categorically denies.
The diplomatic flurry saw Fabius arrive back in Vienna on Saturday to rejoin Germany's Frank-Walter Steinmeier.
Late Saturday, they sat down with Kerry, British Foreign Secretary Philip Hammond as well as Russian and Chinese representatives to try to inject new momentum into the talks.
TIME TO CALL IT QUITS?
Kerry also spoke by phone with Russian Foreign Minister Sergei Lavrov.
With many politicians on both sides of the Atlantic openly questioning whether it is time to walk away, the US State Department said that point had not yet been reached.
The terms of an interim accord agreed in November 2013 have been extended until Monday to give the negotiators more time to strike an accord.
"There's still work that needs to be done, but they're going to continue working through the weekend," said Mark Toner, deputy State Department spokesman.
The toughest problems have been left to last, including a mechanism for lifting interlocking EU, US and UN sanctions.
A new headache emerged in recent days, when the Iranian delegation insisted that a UN arms embargo must be lifted once a deal is reached.
Expert Kelsey Davenport, from the Washington-based Arms Control Association, warned: "Now is not the time for brinksmanship or a hardening of positions."
"This is an historic moment and there could be serious repercussions if negotiators fail to seize this opportunity to get a good deal," she told AFP.