VIENNA (AFP) - Iran and world powers embark on Tuesday on the Herculean task of transforming an interim nuclear deal into a long-term accord satisfying all sides and silencing talk of war for good.

After a decade of failure and rising tensions, US President Barack Obama has put the chances of such an agreement at not more than "50-50", while Iranian Foreign Minister Mohammad Javad Zarif has predicted "difficult" discussions.

The scheduled three-day meeting in Vienna between senior diplomats from Iran and the United States, China, Russia, Britain, France and Germany - the P5+1 - is the first in what is expected to be a series of tricky encounters in the coming months.

It comes after foreign ministers struck a deal in Geneva on November 24 that saw Iran agree to curb - for six months - some of its nuclear activities in exchange for minor relief from painful sanctions.

That agreement, which came into force on January 20, extends the theoretical "break-out time" needed by Iran - which denies seeking the bomb - to produce enough highly-enriched uranium for a nuclear weapon.

In return Iran is due to get over the six months some US$6-7 billion (S$7.5-8.8 billion) in sanctions relief, including US$4.2 billion in assets frozen in overseas bank accounts.

But Iran's nuclear freeze is only temporary - although it can be extended - and the bulk of UN and Western sanctions remain, continuing to deprive Iran of billions of dollars in oil revenues every week.

Under the "comprehensive" deal now being sought, which the parties aim to conclude and commence implementing by November, the powers will want Iran to scale back its activities permanently.

In exchange, Iran would see all UN Security Council, US and EU sanctions lifted.

The deal might include the closure of Iran's Fordo facility, slashing the number of centrifuges enriching uranium, cutting its stockpile of fissile material and altering a new reactor being built at Arak, diplomats say.

This, plus tighter UN inspections, would not remove entirely Iran's capability to get the bomb but would make it substantially more difficult.

According to Mr Obama, it would be "impossible".

"Your enrichment capacities are incompatible with your civilian nuclear programme, we are saying to them ... They don't fit with your statement that you have no military objectives," one Western envoy involved in the Vienna talks said.

But how such a deal goes down with hardliners in the United States and in Israel, the Middle East's sole if undeclared nuclear power, and also with Sunni Arab monarchies in the Gulf, remains to be seen.

"I think both sides would be willing to make compromises to meet the bottom lines of the other side," Richard Dalton, the former British ambassador to Tehran now at think-tank Chatham House, told AFP.

"The trouble is that both sides have hard men outside the negotiating room who have to be satisfied."

Iranian President Hassan Rouhani, a relative moderate whose election last year has helped thaw relations with the West, for now retains the backing of supreme leader Ayatollah Ali Khamenei.

Mr Rouhani called last week for the talks to be "fair and constructive", while Teheran has laid out a series of "red lines" including refusing to close down any nuclear facilities or to stop medium-level enrichment.

"I am not fully sure whether Khamenei himself is fully committed to this process yet," Siavush Randjbar-Daemi, Iran and Middle East lecturer at Manchester University, told AFP.

"Geneva really was a stop gap, a band-aid solution that didn't really heal the wounds."