Six world powers and Iran groped Saturday for ways to bridge wide gaps in marathon nuclear negotiations two days ahead of a deadline.
At stake in the Austrian capital Vienna is a historic deal in which Iran would curb its nuclear activities in exchange for broad relief from years of heavy international economic sanctions.
It could end a 12-year standoff with the West that has even raised the threat of Israeli military strikes against Iranian nuclear facilities.
US Secretary of State John Kerry was consulting by phone Saturday with his international partners as well as colleagues back in the United States, aides reported, after Washington said "serious gaps" remained in the talks.
Iranian sources said Iranian Foreign Minister Mohammed Javad Zarif would meet Kerry later Saturday for a fourth round of talks here in three days, but there was no immediate confirmation of this from the US State Department.
"The gap remains big...There now needs to be a political decision," an Iranian source told AFP on condition of anonymity, putting the onus on the world powers.
The United States also said hurdles remained high.
"We are running against the clock. Obviously, the deadline is Monday, and our folks there are working furiously to meet it... Serious gaps do remain," White House spokesman Eric Schultz said in Washington.
But he warned Washington would not settle for a bad deal.
The gaps were so wide that Britain and Iran have raised the possibility that a final deal would not be reached by Monday's deadline, and that the negotiations could be extended.
- 'Political will missing?' -
The United States, Russia, China, Britain, France and Germany have been locked in talks with Iran since February to turn an interim accord reached a year ago into a lasting agreement by November 24.
Such a deal is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities.
The Islamic Republic hotly denies such an aim and insists its programme is entirely peaceful.
A source close to the Iranian delegation told AFP the negotiators aimed for something short of a comprehensive nuclear agreement, seeking instead a deal on "a general framework" whose details would be filled in later.
Signup to our newsletter and follow us on Facebook and Twitter!
"There is no other scenario possible at this stage," the source added.
British Foreign Secretary Philip Hammond had raised the possibility of a delay even if Kerry had said he intended to seal a deal here.
Hammond, who left Vienna on Friday, said there was still a "very significant gap" while French Foreign Minister Laurent Fabius called on Iran to "seize this opportunity."
Aides said Fabius returned to Paris on Saturday but would be back in Vienna on Sunday.
German Foreign Minister Frank-Walter Steinmeier was due for talks in Vienna with Kerry, US officials said. Zarif was also to meet the German top diplomat, Iranian sources added.
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".
Kerry later "updated Foreign Minister Lavrov on the state of negotiations" by phone, the US official said.
Iran believes that the onus is on the side of the world powers to compromise. Zarif and President Hassan Rouhani both warned the powers not to sink the talks with "excessive demands".
- Complex deal -
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.
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.
Despite Israeli threats of a strike on Iran if world powers fail to halt its nuclear drive at talks entering an endgame in Vienna, experts see the sabre-rattling as largely diplomatic brinkmanship.
With Monday's deadline for a lasting agreement fast approaching, Israel has been keeping up pressure on the six world powers.
Israel, the region's sole if undeclared nuclear-armed state, sees itself as the first potential target if Iran develops an atomic bomb, and has warned against striking what it describes as a "bad deal" with Tehran.