Diplomats arriving in Austrian capital warned that failure to agree end to deadlock over Iran’s nuclear programme is not an option

Iran, the US and other world powers meeting in Vienna this week are close to a historic, comprehensive agreement that could bring a permanent end to 12 years of deadlock over Iran’s nuclear programme.

With a deadline for the talks looming in a week’s time, diplomats are converging on the Austrian capital for the last stretch of marathon negotiations beginning Tuesday, with the outcome still in the balance.

Compromises have been found on previously contentious issues, and detailed text for different versions of a final deal has been drafted.

Some diplomats describe their work as 95% done, pending political decisions to be made in national capitals over Iran’s capacity to enrich uranium over the next few years, and the sequence in which international sanctions are lifted.

Several leading arms-control experts have argued that the residual obstacles are more political than substantial, determined by the need of President Barack Obama’s administration and President Hassan Rouhani’s reformist government in Iran to reassure conservatives at home, rather than by the actual requirements of Iran’s nuclear energy programme or genuine nonproliferation concerns.

There are also differences among the six-nation group involved in the negotiations with Iran. France has consistently been more opposed to nuclear concessions than the other five (the US, UK, Germany, Russia and China).

John Kerry, the US secretary of state, flew to Paris on 4 November for talks with Laurent Fabius, the French foreign minister, seeking assurances that he would not make a public intervention in the last few days of negotiations. In the closing stages of talks over the 2013 interim deal, Fabius warned against western concessions, saying Paris would not play along with a “fool’s game”.

Accounts vary as to whether Kerry was able to secure a guarantee from Fabius not to break ranks in the eleventh hour of talks.

“For Fabius, the ties with the Gulf Arabs – Saudi Arabia and Qatar – are much more important economically, and for French jobs in the next few years, than Iran,” said a French source familiar with the discussions. The Sunni monarchies in the Gulf are as opposed as Israel to western endorsement of an Iranian nuclear programme on any scale.

The consequences of a collapse in the negotiations could be serious and rapid. The US Congress is poised to impose fresh sanctions on Iran, and after the Republican capture of the Senate in this month’s elections it will be hard for Obama to sustain a veto on new punitive measures. In response, hardliners in Tehran are likely to demand an end to the partial freeze on the Iranian programme negotiated in an interim agreement a year ago. Mutual escalation could quickly push the 12-year nuclear standoff back to the brink of war. Israel has repeatedly threatened to take military action if diplomacy fails to contain Iran’s nuclear aspirations.

“I think extension is the least likely scenario because of the domestic political ramifications for all sides,” said Reza Marashi, a former state department official, who is research director of the National Iranian American Council in Washington. “In the words of the negotiators themselves: failure is not an option.”

Given the high stakes, all sides at the Vienna talks will be extremely reluctant to break off negotiations if a complete agreement is not reached by 24 November, the deadline agreed in the interim deal, at Geneva a year ago. One option would be to announce a framework agreement, leaving gaps to be worked out later, or simply extend the talks. But neither option would be politically sustainable for long without proof of genuine progress. Congress is already sceptical of the talks, claiming Iran is playing for time.

“It’s now or never,” said Jim Walsh, an expert on the Iranian nuclear programme at the Massachusetts Institute of Technology. “If you put this off six months, does Obama come back stronger, able to deliver a deal? Does Rouhani? No.

“My hope is that, at the last moment, they stare into the abyss and realise the consequences of not doing a deal now are very, very bad.”

Ali Vaez, an analyst at the International Crisis Group, suggested that, at most, the parties would have the remaining two months of the lame duck Congress to resolve the last remaining issues.

Those outstanding obstacles, enrichment capacity and the sequence of sanctions relief, have long been the most politically charged and difficult issues on the table.

The west is offering a temporary suspension of some US sanctions through a presidential waiver of measures imposed by Congress, along with the unfreezing of blocked Iranian assets around the world. The lifting of major oil and banking sanctions would be left until later. Tehran wants the permanent lifting of the major sanctions early in the lifetime of a deal, including those imposed by the UN security council.

Iran has 19,000 centrifuges installed in two enrichment plants. Of those, 10,200 first-generation machines are in operation. The west would like that cut to fewer than 4,000 (the limits are measured in overall capacity, so a smaller number of more advanced centrifuges would be allowed), with the aim that it would take Iran a year to amass enough fissile material for a warhead, if Tehran took the decision to make a weapon.

For its part, Iran does not want to cut capacity, and wants assurances that it will be allowed to expand in line with its nuclear energy needs.

The rigid positions adopted by both sides have come under criticism from arms control experts. Daryl Kimball, of the Arms Control Association, pointed out that Russia had guaranteed the supply of fuel for Iran’s reactor at Bushehr and for at least two new reactors that Moscow and Tehran had agreed to build last week. “Iran’s practical need in the near term is close to zero,” said Kimball, the head of the association. “Tehran is within sight of a compromise on a linchpin issue. It would be a tragic and historic mistake if Iran passed up the chance for this agreement, if they refused to compromise in order to hold on to a couple of thousand old-fashioned, inefficient centrifuges.”

Several nonproliferation specialists have also questioned the US-led insistence that Iran’s “breakout capacity” (the time it would take it to make a warhead) is kept to a year.

Jeffrey Lewis, at the Monterey Institute of International Studies, said that if Iran wanted to make nuclear weapons, it would do so at covert sites rather than use facilities under constant international monitoring. “The Iranians are not going to use a declared facility. That would provoke an immediate crisis. Maybe they were be able to build one or two bombs but they would get hit,” said Lewis, who runs the ArmsControlWonk.com website. “If they were going to do it, they would do it in a covert facility. So everything we do should be focused on verification, not on numbers of declared centrifuges.”

The guiding principle over nine months of talks on a comprehensive nuclear agreement has been that “nothing is solved until everything is solved”, reflecting the intertwined nature of the issues. But sources close to the talks say several difficult problems are close to a solution.

• Lifetime of the deal – Iran originally wanted it to last no more than three years. The west wanted a 20-year deal. A likely compromise is in the eight- to 10-year range.

• Cooperation with an International Atomic Energy Agency inquiry into alleged past Iranian development work on nuclear weapons – the IAEA would have to confirm full cooperation before the last major sanctions are lifted.

• Heavy water reactor being build in Arak, central Iran – this would be redesigned to produce less plutonium as a byproduct. Iran would undertake not to build a reprocessing facility for extracting plutonium.

• Underground enrichment site at Fordow – a small-scale research-and-development centre would be allowed under strict IAEA supervision

• Transparency – Iran would accept a permanent IAEA presence at its nuclear facilities, while the agency would be able to inspect undeclared sites for signs of nuclear activity and monitor centrifuge-making plants.

The negotiators could decide to bank those gains as the deadline of Monday night next week looms, issuing a statement on progress while asking for more time for the remaining obstacles – but that may not be enough to convince conservatives in the US and Iran to accept an extension.