Iran and world powers' high-stakes nuclear talks enter a critical fifth round in Vienna on Monday, with both sides still far apart on crucial issues five weeks before a deadline for a deal.

The five permanent members of the UN Security Council plus Germany want Tehran to scale back its nuclear activities, while Iran wants all UN and Western sanctions to be lifted.

This long hoped-for accord would be aimed at once and for all silencing fears that Tehran might develop nuclear weapons, and averting a slide into international conflict.

It also remains to be seen whether possible cooperation between Iran and the United States on the Iraq crisis will help the old foes find common ground in Vienna.

President Hassan Rouhani said Saturday that Iran would consider helping Washington if the US took action to stem a lightning offensive by Sunni militants in Iraq, but that there were still "differences" in the nuclear talks -- some of them "substantial".

Both sides caution that there is a long way to go as negotiators confront the same sticking points that have dogged diplomatic efforts for the past decade.

"There is still lots of work to do. There are glimpses of outlines of solutions on different issues but it is all very fragile," said a Western diplomat involved in the negotiations.

"On the more important issues, there haven't even been glimpses of solutions."

- Uranium enrichment -

The many thorny issues to be resolved in what would be a fiendishly complex deal include the duration of the mooted accord and the pace and timing of any sanctions relief.

Others include Iran's partially-built Arak nuclear reactor, which could give it weapons-grade plutonium, and allegations of past atomic weapons research.

But the gorilla in the room remains uranium enrichment, a process that can produce nuclear fuel but also, when highly purified, the core of an atomic bomb.

Iran wants to massively increase the number of centrifuges, saying it needs them to produce the fuel for a future fleet of civilian nuclear plants.

The West says that such facilities are years if not decades away from being built, fearing that Iran's real aim is to use the enriched uranium for a bomb -- something Tehran denies.

French Foreign Minister Laurent Fabius said last week that the West wants Iran to slash the number of centrifuges -- the machines used to enrich the metal -- to "several hundred" from the current 20,000.

"We are not even in the same ballpark," said Fabius. "Wanting hundreds of thousands of centrifuges is pointless unless you want the bomb."

- Extra time -

Under an interim deal struck in November, Iran agreed to freeze certain nuclear activities for a period of six months in return for minor sanctions relief.

This comes to an end on July 20 but it can be renewed if needed -- and if both sides agree.

Experts say such an extentions is likely already under discussion.

"The powers and Iran have been drafting documents in preparation for an eventual extension for a long time," Mark Hibbs, of the Carnegie Endowment for International Peace, told AFP.

US President Barack Obama would much prefer to get a deal by July 20 in order to fend off accusations that Iran is merely buying time ahead of midterm US elections in November.

This has been the long-standing accusation made by Israel, the Middle East's sole if undeclared nuclear-armed state, which refuses to rule out bombing Iran's nuclear facilities.

But Rouhani argues Tehran is "serious in the negotiations" -- and that an accord is possible this time.

"It will be in the interest of everyone if a deal is signed in the next five weeks," he said.