World leaders are at the negotiation table in Vienna, where U.S. Secretary of State John Kerry says "big gaps" remain in talks over Iran's nuclear program. (Reuters)

Negotiators working to slow Iran’s nuclear program and ease sanctions pressed forward with talks Saturday amid indications that they are at an impasse with two days left before a deadline for an accord.

In brief remarks before going into a closed-door meeting, Secretary of State John F. Kerry characterized the talks with Iran as “difficult” and marked by “serious gaps.” His German counterpart, Frank-Walter Steinmeier, described the remaining time before Monday’s deadline as “the hours of truth.”

Before reporters were escorted out of the room, Kerry was overheard making small talk with Steinmeier, saying “We’re stuck,” and “We were ready to go.”

Although the context was not entirely clear, the offhand remarks seem to have been referring to the confusion that swirled around itineraries Friday. Both Kerry and Iranian Foreign Minister Mohammad Javad Zarif announced plans to leave Vienna for consultations, Zarif to Tehran and Kerry to Paris. But they ended up staying in Vienna to meet for over two hours. Zarif told the Iranian media that he did not consider the plan espoused by the United States and the five other nations that are its negotiating partners strong enough to run past Tehran.

Kerry acknowledged Saturday that the talks with Iran were a tough slog.

Iran and the United States have intensified efforts to overcome deep divisions in talks on Tehran's nuclear program, days ahead of a deadline for agreement. (Reuters)

“We’re working hard,” he told reporters. “We hope we’re making careful progress. But we have big gaps. We still have some serious gaps, which we’re working to close.”

Steinmeier made reference to the ticking clock leading up to Monday’s deadline, when an interim agreement that froze Iran’s nuclear program expires.

“This is the weekend, these are the hours of truth,” Steinmeier said. “And we have to check now if Iran is really ready to move in the right direction. The only criterion for a deal about the nuclear issue . . . is the main question: If Iran is really ready to renounce every research development working in the direction of getting nuclear weapons.”

Their remarks contributed to a sense of pessimism about the fate of the talks, which started more than a decade ago but picked up steam last year.

According to a Western diplomat, huge differences remain on sanctions relief and how much uranium-enrichment capacity Iran would be permitted.

“It’s hard to see how, technically, they get there,” the diplomat said.

Nevertheless, the negotiators are discussing new ideas about how to breach the gap, and continue to “chip away” at areas of disagreement, according to the diplomat.

German Foreign Minister Frank-Walter Steinmeier, left, speaks as he and Secretary of State John F. Kerry address the media on Saturday before reentering closed-door nuclear talks with Iran in Vienna. (Ronald Zak/AP)

Nobody wants to see the talks collapse, which could set off an escalation in a region roiled by conflict. One possible scenario is that the two sides agree on principles and extend the interim accord so the experts can work out the details in coming weeks or months.

Iranian President Hassan Rouhani sounded one of the few notes of optimism in a tweet posted Saturday on his English language account: “If the parties in #IranTalks have necessary political will 4agreement, the condition will be ripe 4clinching a final agreement.”

Kerry met Saturday afternoon for the fourth time in two days with Zarif and the European Union representative, Catherine Ashton.

He spent the morning working the phones, speaking with foreign ministers from Turkey, Canada, Kuwait, Qatar, Bahrain and the United Arab Emirates and meeting with the French foreign minister. In between Kerry managed to make a quick visit to one of Vienna’s outdoor Christmas markets.

Kerry also called Israel to brief Prime Minister Benjamin Netanyahu.

Israel has argued against a deal that would leave Iran with any capacity to enrich uranium. The United States has modified its stance, and wants Iran to reduce its uranium-enriching centrifuges so it would take Iran at least a year to get enough material for a nuclear weapon. Iran has resisted, saying it wants the material for reactors still unbuilt, and the material would be used for civilian purposes.

Iran has chafed at some of the other proposed restrictions, a position driven as much by Iranian politics and dignity as by science. The negotiators — the United States, France, Britain, China, Russia and Germany — want enough Iranian cutbacks to guarantee nonproliferation. They propose easing international and American sanctions against Iran temporarily and gradually so they can be resumed if Iraq acts covertly. Iran seeks sanctions relief that is more permanent and swift, in part so the deal cannot be undone by a GOP-led Congress.

And while Iran is willing to submit to monitoring of its nuclear program, it wants an end point a few years out. The six-nation negotiating team has proposed a much longer time frame.