Iran and world powers ratcheted up the rhetoric Tuesday as they entered a final round of nuclear talks six days before a deadline for a deal, with still-considerable differences dogging the negotiations.

Iranian Foreign Minister Mohammad Javad Zarif warned on arrival in Vienna that an accord would only happen if the five permanent members of the UN Security Council plus Germany make no "excessive demands".

"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.

But US Secretary of State John Kerry, in London but expected in Vienna in the coming days, said it was "imperative that Iran works with us with all possible effort to prove to the world that the programme is peaceful".

"This is a very critical week obviously in Iran negotiations," Kerry said. "We hope we get there but we can't make any predictions."

In a joint news conference, British Foreign Secretary Philip Hammond called for more "flexibility by the Iranians to convince us that their intentions in their nuclear programme are entirely peaceful".

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 civilian activities.

It could resolve a 12-year standoff over Iran's atomic programme, silence talk of war and help normalise Iran's fraught relations with the West after 35 years of mistrust and antagonism.

It could also boost Iran's economy, improve the lives of ordinary Iranians and mark a rare foreign policy success for US President Barack Obama, five years after he offered Tehran an "outstretched hand".

US and Iranian negotiations are under domestic pressure not to give too much away, however, while Israel -- the Middle East's sole if undeclared nuclear-armed power -- and others in the volatile region are sceptical.

- Devil in the detail -

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.

At present Iran could use its existing infrastructure to produce enough weapons-grade uranium for one bomb in a few months, although any such "breakout" attempt would be detected very quickly.

And 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 of centrifuges slashed, saying Iran has no such need at present, something that would extend the "breakout" period to at least a year.

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.

"There is virtually no possibility that a complete deal will be concluded by November 24," former top US diplomat Robert Einhorn, now an expert with the Brookings Institution, told AFP, predicting another extension of "several more months".

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 the deadline.

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

Zarif held a working lunch in the Austrian capital Tuesday with the powers' lead negotiator, former EU foreign policy chief Catherine Ashton.

Further meetings included one between all six powers and Iran and US-Iran bilateral discussions, diplomats said.