France's insistence on tough terms on Iran surfaced earlier. French Foreign Minister Laurent Fabius spoke of "several points that ... we're not satisfied with compared to the initial text," telling France-Inter Radio his nation does not want to be part of a "con game." He did not elaborate, but it appeared France wanted tougher constraints on a reactor that will make plutonium when completed and also on parts of Iran's enrichment program.

Iranian state TV strongly criticized the French position, calling France "Israel's representatives at the talks," while Iran's IRNA news agency cited Iranian President Hassan Rouhani as urging world powers to reach a deal. The French position dampened much of the optimism surrounding the opening of the talks Thursday.

As the day dragged on Saturday, comments by Zarif increased skepticism that the two sides would agree on the full contours of a first-step deal at the current negotiating round. "There are differences," Zarif told Iranian state TV, adding that if open questions remained after Saturday, the talks would reconvene within a week to 10 days.

Iran's Arak reactor southeast of Tehran could produce enough plutonium for several nuclear weapons a year once it goes online. Beyond differences over that part of Iran's nuclear program, Fabius said there was disagreement over efforts to limit Iran's uranium enrichment to levels that would require substantial further enriching before they could be used as the fissile core of a nuclear weapon.

French Foreign Ministry spokesman Romain Nadal said the powers negotiating with Iran were broadly united and that France wanted "the international community to see a serious change in the climate" of talks with Iran. "There have been years of talks that have led to nothing," Nadal said, alluding to the need for tough terms on Iran.

Iran insists it is pursuing only nuclear energy, medical treatments and research, but the U.S. and its allies fear that Iran could turn this material into the fissile core of nuclear warheads. Iran currently runs more than 10,000 centrifuges that have created tons of fuel-grade material that can be further enriched to arm nuclear warheads. It also has nearly 440 pounds (200 kilograms) of higher-enriched uranium in a form that can be turned into weapons much more quickly.

Experts say 550 pounds (250 kilograms) of that 20 percent-enriched uranium are needed to produce a single warhead. Iran says it expects Arak, the plutonium producing reactor, to be completed and go online sometime next year. It would need additional facilities to reprocess the plutonium into weapons-grade material, and the U.N.'s nuclear agency monitoring Iran's atomic activities says it has seen no evidence of such a project.

Fabius said Iran opposes suspending work on Arak while nuclear negotiations go on. He said France considers suspension absolutely necessary. Iran also is being asked to blend down "a great part of this stock at 20 percent, to 5 percent," Fabius said.

Uranium enriched to 5 percent is considered reactor fuel grade, and upgrading it to weapons-level takes much longer than for 20 percent enriched uranium. Fabius suggested that the six powers were looking for an Iranian commitment to cap future enrichment at 5 percent.