For the six global powers in talks with Iran, the goals have always been clear — blocking Iran’s path to a nuclear bomb while eventually lifting sanctions that have hobbled its economy and caused hardship for the Iranian people.

But as negotiators gather in Vienna this week in one last push for a deal, decades of mutual mistrust and years of Iranian obfuscation on its nuclear program may be proving too much to surmount.

Three days of talks in Oman last week produced no leap forward, with a deadline for an accord fast approaching. Now, negotiators and nuclear experts are sounding increasingly skeptical that a comprehensive deal is likely or even possible by the Nov. 24 deadline.

Iranian and Russian negotiators, who recently offered assurances that there was plenty of time to strike a deal, acknowledged last week that talks may be extended. President Obama warned, “We may not be able to get there.”

“It’s pretty clear that, barring a miracle, there’s not going to be a comprehensive deal struck on the 24th of November,” said Mark Fitzpatrick, a former U.S. diplomat who is director of the nonproliferation and disarmament program at the International Institute for Strategic Studies. “The two sides just remain too far apart, and the amount of horse-trading that would be required for a deal is too complex to get it all done in time, even if both sides really wanted to.”

U.S. Secretary of State John Kerry, right, and Iranian Foreign Minister Mohammad Javad Zarif, shake hands during a meeting in Muscat, Oman on Sunday, Nov. 9, 2014. (Nicholas Kamm/AP)

Under an interim agreement, Iran has frozen the buildup of its nuclear capacity. Without an extension, that pact dies Nov. 24. If Iran chose to walk away and resume expanding its nuclear capacity, it could lead to a military confrontation that could escalate quickly beyond Iran’s borders in a region racked by war, violence and political instability.

The best-case scenario is that both camps agree to keep talking.

“What is still possible is a breakthrough that could justify adding more time to the clock,” said Ali Vaez, a scientist who focuses on Iran for the International Crisis Group.

High hurdles

The elusiveness of a final agreement after a year of intense negotiations indicates the seriousness with which both sides view the talks and the high hurdles that remain.

The negotiations had been sputtering along in fits and starts for almost a decade, then took on new life last year after a moderate, Hassan Rouhani, was elected president of Iran. He promised to rid the country of the bruising sanctions that have sent the economy into a tailspin, which has been compounded by plummeting oil prices.

The negotiators — Iran and the five permanent members of the U.N. Security Council plus Germany — have been tight-lipped on details, many of which are mind-numbingly complex and technical. But three broad sticking points have emerged.

The six negotiating partners, primarily the United States, want Iran to slash its 19,000 uranium-enriching centrifuges to levels low enough that monitors would notice if Iran cheated and tried to make a nuclear bomb. Iran says its nuclear industry is for civilian energy production and medicine, and it has balked at cuts.

Skeptics in the United States and elsewhere want to know whether Iran once had a nuclear weapons program, as many believe, and if it is ongoing. But Iran has not provided enough access or information for the International Atomic Energy Agency to conclusively rule on a possible military dimension to Iran’s program.

For its part, Iran appears to be insisting on a rapid lifting of all sanctions. American and European diplomats want the measures to be removed gradually, after Iran shows it is cooperating.

“If you can’t agree on key parameters, that will raise legitimate questions on whether a final deal can be concluded,” said Robert Einhorn, a senior fellow with the Brookings Institution who specializes in arms-control and nonproliferation issues. “It’s going to take a lot of cooperation by Iran, and that’s going to take awhile. It’s unrealistic to ask Congress to vote to lift sanctions before Iran does that.”

Wild card in Washington

Congress is one of the wild cards in any delay. Many Republican and Democratic members believe that sanctions helped nudge Iran to the negotiating table and that they are being eroded by an administration ­eager for a legacy-building deal. Some already are talking about imposing more sanctions as leverage to make Iran more transparent and cooperative with IAEA monitors.

When the GOP gains control of Congress in January, Republicans are likely to be less willing to wait for the talks to evolve, particularly if the price of keeping Iran at the table is a partial easing of some sanctions.

George Perkovich, who focuses on nuclear and nonproliferation issues at the Carnegie Endowment for International Peace, said more sanctions by Congress could push the Iranians to give up on talks entirely.

“It would create a lot of pressure within Iran from those who say, ‘Look, the United States is totally about regime change, it’s all they’ve ever been after,’ ” he said. “In my opinion, it makes more sense to let Iran be the ones who escalate, so the rest of the world sees them as provocateurs rather than the U.S. being the one seen as escalating things.”

Olli Heinonen, who works on nuclear nonproliferation issues at Harvard University’s Kennedy School of Government, said Iran may be stalling out of a sense of overconfidence. He said some in Iran may believe they have the upper hand in talks because the United States and its partners are preoccupied with the spread of Islamist militancy elsewhere in the Middle East.

“This can’t go on forever,” he said of the talks, predicting they will be extended for a few months more. “It will be on the 24th, late at night, or early in the morning of the 25th, when we find out. It’s going to be a long, long night. I wouldn’t be surprised if the clocks in Vienna are stopped for a while.”