Iran and six world powers get down to business in Vienna Wednesday, groping for the elusive magic formula to secure a milestone nuclear deal that satisfies hardliners in Tehran and Washington.

The clock will be ticking though on the second day of a final round of talks, with the deadline for Iran and the five permanent members of the UN Security Council plus Germany on Monday.

"This is a very critical week obviously in Iran negotiations," US Secretary of State John Kerry, expected in the Austrian capital later in the week, said in London on Tuesday.

"We hope we get there but we can't make any predictions."

Iranian Foreign Minister Mohammad Javad Zarif, arriving in Vienna on Tuesday, said that a deal was "possible" and that if the talks fail it will be because the six powers wanted too much.

"If, because of excessive demands... we don't get a result, then the world will understand that the Islamic Republic sought a solution, a compromise and a constructive agreement and that it will not renounce its rights and the greatness of the nation," Zarif told Iranian media.

Kerry though, who held the latest in a string of meetings with Zarif in Oman last week, put the onus on Iran.

"It is imperative that Iran works with us with all possible effort to prove to the world that the programme is peaceful," Kerry said.

British Foreign Secretary Philip Hammond echoed his comments, calling for more "flexibility by the Iranians to convince us that their intentions in their nuclear programme are entirely peaceful".

- Drums of war -

The landmark accord being sought by Monday's deadline, after months of negotiations, is aimed at easing fears that Tehran might develop nuclear weapons under the guise of its civilian activities.

It could resolve a 12-year standoff, silence talk of war, help normalise Iran's relations with the West, boost the beleaguered Iranian economy and mark a rare foreign success for US President Barack Obama.

In order to make it virtually impossible for Iran to assemble a nuclear weapon, the US, China, Russia, Britain, France and Germany (the P5+1) want Iran to scale down its nuclear programme.

Iran, which insists its nuclear aims are exclusively peaceful despite failing to declare parts of its programme in the past, wants painful sanctions lifted.

Some areas appear provisionally settled. But the big problem remains enrichment, rendering uranium suitable for power generation and other peaceful uses -- but also, at high purities, for a weapon.

Iran wants to ramp up massively the number of enrichment centrifuges in order, it says, to make fuel for a fleet of future reactors.

The West wants the number slashed, saying Iran has no such need in the foreseeable future.

Other thorny issues are the duration of the accord and the pace at which sanctions are lifted, an area where Iranian expectations are "excessive", one Western diplomat said.

- Another extension? -

Given the differences, many analysts expect more time to be put on the clock.

The alternative -- walking away -- would be "catastrophic," Arms Control Association analyst Kelsey Davenport said.

"Given the political capital that both sides have invested... it would be foolish to walk away from the talks and throw away this historic opportunity," Davenport told AFP.

For now though, with another extension presenting risks of its own -- fresh US sanctions, not least -- officials insist that they remain focused on getting the job done in time.

"An extension is not and has not been a subject of conversation at this point," a senior US official said late Monday.