World powers huddled again Saturday behind closed doors on the 15th day of negotiations seeking a deal to curb Iran's nuclear ambitions, with no sign of any end to the nail-biting deadlock.

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 if the negotiations could be formally extended again in a bid to end the current deadlock.

Iran and the so-called the 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 about the reasons behind the stalemate, Kerry Friday had offered a glimmer of hope that some progress may be being made saying some outstanding issues had been resolved.

But the top US diplomat emerged from almost 90 minutes of fresh talks Saturday morning with his Iranian counterpart Mohammad Javad Zarif and almost immediately 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 in an indication of how difficult the remaining issues are.

France's Foreign Minister Laurent Fabius also arrived back in the Austrian capital Saturday to rejoin Germany's Frank-Walter Steinmeier, while their British counterpart was expected back too. It remained unclear when the Russian and Chinese foreign ministers would return to the city.

With many politicians on both sides of the Atlantic openly questioning whether it was time to walk away, Mogherini tweeted her thanks to the Austrian people for their "prolonged hospitality.

"Vienna, still working day and night on #IranTalks," she said in her Tweet.

The toughest problems have been left to last, including a mechanism for lifting interlocking EU, US and UN sanctions, as well as ways to ensure Iran can have a peaceful nuclear programme for its own energy needs.

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, something that Western countries are wary about given Iran's alleged role in several Middle East conflicts.

In turn Iranian officials blamed the Americans for throwing a spanner in the works saying that after a video conference with US President Barack Obama on Wednesday, the US delegation had changed some of their positions.

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.

"At the same time, these last decisions will not get easier with time."