"We're working hard," Kerry said in Vienna, "and we hope we're making careful progress, but we have big gaps, we still have some serious gaps, which we're working to close."
Kerry, who on Friday (November 21) postponed a trip to Paris to remain in Vienna, met Iranian Foreign Minister Mohammad Javad Zarif on Saturday afternoon, their fourth meeting in three days.
German Foreign Minister Frank-Walter Steinmeier, in the Austrian capital since Saturday midday, called this final weekend of talks, after months of negotiations, a "moment of truth".
At stake 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 that has even raised the prospect of Israeli military strikes on Iranian nuclear facilities. Kerry spoke to Israel's Prime Minister Benjamin Netanyahu Saturday by phone.
"The gap remains big... There now needs to be a political decision," an Iranian source said on condition of anonymity, putting the onus on the world powers to make concessions.
A European source in the talks said Saturday there has been "no significant progress" and "the chances of getting a deal are pretty reduced".
"In order to get a deal the Iranians will have to budge in a rather substantial manner," he said. Discussions about an extension could begin as early as today (November 23), he said.
Many experts believe that the deadline may be extended, as happened with an earlier cut-off point of July 20, but officials insist that this is not on the table – yet.
However a senior US official said late yesterday that the aim remained getting a deal by Monday night (November 24) "but we are discussing both internally and with our partners a range of options".
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.
British Foreign Secretary Philip Hammond and his French counterpart Laurent Fabius also joined the talks on Friday. Both have since left but were expected to return.
It was unclear when or whether Russian Foreign Minister Sergei Lavrov, a key player in the talks, might also arrive.
Earlier this week the head of the Russian delegation said Lavrov would come only if there was serious progress.
Lavrov said from Moscow on Friday that "all the elements are already on the table" for a deal and that all that was missing was "political will".