VIENNA (AFP) ― Nuclear talks between Iran and world powers move to the next level Tuesday as negotiators begin work on an ambitious lasting accord to silence for good fears about Tehran’s atomic ambitions.

Success might help put Iran and Washington on the road to normalizing relations 35 years after the Islamic revolution and bear fruit in other areas, not least in Syria. Failure could lead to conflict.

Expectations were not high, however, ahead of the scheduled three-day Vienna meeting between Iran and the United States, China, Russia, Britain, France and Germany, expected to be the first in a series of tricky encounters.

A day before, Iran’s supreme leader Ayatollah Ali Khamenei said he was “not optimistic” and even added that he expected the talks to “lead nowhere” ― although he also said he was “not against them.”

“It is probably as likely that we won’t get an agreement as it is that we will,” said one senior U.S. administration official in Vienna for the talks.

“But these negotiations are the best chance we have ever had for diplomacy to solve this most pressing of national security challenges.”

Iran has long been suspected of seeking nuclear weapons, despite its denials, while the United States and Israel ― widely assumed to have a formidable nuclear arsenal itself ― have never ruled out military action.

On Nov. 24 in Geneva foreign ministers from the seven countries struck a deal widely hailed as an enormous breakthrough after a decade of failed diplomatic efforts and rising tensions.

Under this accord, which took effect on January 20, Iran scaled back certain nuclear activities in exchange for minor relief from painful sanctions and a promise of no new sanctions.

For the first time the West accepted Iran enriching uranium, a process producing nuclear fuel but potentially also material for a bomb, having previously demanded a total suspension.

But the freeze only lasts until July 20 ― although it can be extended ― and experts say that success in Geneva came at the price of postponing discussion on the really difficult issues.

“Geneva really was a stop gap, a band-aid solution that didn’t really heal the wounds,” Siavush Randjbar-Daemi, Iran and Middle East lecturer at Manchester University, told AFP.

Under the “comprehensive” solution that the parties aim to sew up by November, the six powers want Iran to scale back permanently ― or at least for a very long time ― its nuclear program.

This might include closing the Fordo facility, slashing the number of centrifuges enriching uranium, cutting the stockpile of fissile material and altering a new reactor being built at Arak.

This, plus much tighter U.N. inspections, would not remove entirely Iran’s capability to get the bomb but would make it substantially more difficult.

According to U.S. President Barack Obama, it would be “impossible.”

In exchange, all U.N. Security Council, U.S. and EU sanctions on Iran ― which are costing it billions of dollars every week in lost oil revenues, wreaking havoc on the economy ― would be lifted.

But whether Iran will play along remains to be seen, having before the talks set out a number of “red lines” including not dismantling any facilities.

The senior diplomats in Vienna will be well aware that whatever they agree will need to be sold not only to other countries like Israel and the Sunni Gulf monarchies, but also back home.

Obama has to contend with members of Congress threatening more sanctions and demanding ― with Israel ― that nothing short of a total dismantlement of Iran’s nuclear facilities will do.

Iranian President Hassan Rouhani, meanwhile, whose election in 2013 has helped thaw relations with the West, is already on thin ice with hardliners seeking to turn Khamenei against him.