VIENNA - US Secretary of State John Kerry and other foreign ministers engaged Friday in a frenzied day of Iran nuclear talks in Vienna three days before a deadline to strike a mammoth agreement.

Confusion reigned over Iranian Foreign Minister Mohammad Javad Zarif’s plans, with an Iranian source telling AFP he might return to Tehran for consultations - prompting speculation of some progress - only to say later he would stay in Vienna. France and Britain’s foreign ministers joined the negotiations but were due to leave late Friday - as was Kerry, although the announcement of his departure for Paris was made when it appeared Zarif was leaving.

Iran and the six powers - the US, China, Russia, Britain, France plus Germany (P5+1) - have been negotiating intensively since February to turn an interim accord with Iran reached a year ago into a lasting agreement before November 24. Such a deal, after 12 years of rising tensions, is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities - an ambition the Islamic republic has denied.

 British Foreign Secretary Philip Hammond said before leaving the Austrian capital on Friday that there had been a “series of useful discussions” but that there is still “a significant gap”.

“The prize for Iran is huge,” Hammond said. He called for more flexibility from the Iranians and said that “in return we’re prepared to show some flexibility on out side”. Russian Foreign Minister Sergei Lavrov, a crucial player, said from Moscow that “all the elements are already on the table” for a deal and that all that was missing was “political will”. Arriving in Vienna, French Foreign Minister Laurent Fabius, seen as one of the most hardline among the P5+1, had called on Iran to “seize this opportunity”. Most analysts expect Iran and the six powers to decide to extend the deadline. But Kerry on Thursday insisted this was not on the table. “We are not discussing an extension. We are negotiating to have an agreement. It’s that simple,” Kerry said in Paris before he went to Vienna. Hammond had said on Wednesday that he was “not optimistic” the deadline could be met, suggesting that the best hope was for another extension - and only provided there was “some significant movement”.

Iran believes that the onus on the side of the world powers to compromise. Zarif and President Hassan Rouhani both warned the P5+1 not to sink the talks with “excessive demands”.

Iran’s speaker of parliament Ali Larijani meanwhile told Iranian media on Thursday: “We are constantly cooperating (but the other side) is raising the tone.” He added: “We hope that the other side will behave in a rational manner... and won’t take the wrong path.”

Some areas under discussion appear provisionally settled in what would be a highly complex deal that would run for many years, even decades.

But two key issues remain: enrichment - a process that renders uranium suitable for peaceful uses but also, at high purities, for a weapon; and the pace of the lifting of sanctions.

Diplomats say Iran wants all sanctions lifted at once. The six world powers want however to stagger any suspension to be sure that Iran does not renege on its commitments.

Iran wants to massively ramp up the number of enrichment centrifuges - in order, it says, to make fuel for a fleet of future reactors - while the West wants them dramatically reduced.

Kelsey Davenport, an expert at the Arms Control Association, said that it remained possible that Zarif might still return to Tehran, and that it wasn’t necessarily a “bad sign” if he stayed.

“There is still a lot of time until Monday at midnight,” she told AFP.

Comments indicating flexibility on both sides show that “both parties are coming into these final days willing to look at their positions and make some decisions about the remaining tough issues,” she said.