World powers have met again 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 on Saturday 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 on 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 on 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 stand-off, has moved at a snail's pace.