VIENNA - World powers met again Saturday behind closed doors with no end in sight to a nail-biting deadlock in arduous negotiations striving to curtail Iran’s nuclear ambitions in return for sanctions relief.

Despite a warning from US Secretary of State John Kerry that he would not sit at the negotiating table forever, 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, asked whether the negotiations could be formally extended again in a bid to end the deadlock.

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. After a very public blame game over the stalemate, 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 morning with his Iranian counterpart Mohammad Javad 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 in his tweet.

Despite almost two years of negotiations, this round of talks in Vienna, touted as the last push for a historic accord to end a 13-year standoff, has moved at a snail’s pace. 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 — and ministers since June 27 — have been wrangling over how to implement the Lausanne guidelines on the ground. The negotiations have also stumbled over demands that UN nuclear inspectors have access to military sites, amid suspicions that Iran has sought to develop nuclear weapons in the past, allegations Tehran categorically denies.

The diplomatic flurry saw France’s Foreign Minister Laurent Fabius arrive back in Vienna on Saturday to rejoin Germany’s Frank-Walter Steinmeier. British Foreign Secretary Philip Hammond also returned a few hours later and went directly into talks with Kerry. But it remained unclear when the Russian and Chinese foreign ministers would be back.

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 parties are all there, they’re engaged, they’re working through some tough issues. There’s still work that needs to be done, but they’re going to continue working through the weekend. They’re going to be working at this every day,” 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. Taking a few additional days to continue talking is far better than walking away from the table,” she told AFP.