Iran and world powers are engaging in a diplomatic blitz less than 36 hours before a deadline for a nuclear deal, with Tehran signalling openness to extending talks by up to a year.

Officials on both sides insisted however that for the rest of Sunday at least, they are still pulling out all the stops to nail down in Vienna what would be a mammoth agreement by midnight Monday.

However, large gaps appeared to remain in crucial areas of the negotiations involving uranium enrichment and sanctions relief.

The five permanent members of the UN Security Council (the P5+1) have been locked in talks with Iran for months to turn an interim deal struck in Geneva that expires on Monday into a lasting accord.

Such a deal is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities, an ambition the Islamic republic hotly denies.

It could end a 12-year standoff that has hobbled the Iranian economy and prevented a normalisation of Tehran's ties to the West after 35 years in the deep freeze.

US Secretary of State John Kerry, in Vienna since Thursday, was set to hold the latest in a series of meetings later Sunday with Iranian Foreign Minister Mohammad Javad Zarif.

German Foreign Minister Frank-Walter Steinmeier arrived on Saturday, calling the talks a "moment of truth", while his counterparts from Russia, China, Britain and France were also expected by Monday at the latest.

Saudi Foreign Minister Prince Saud al-Faisal also jetted into the Austrian capital, with Kerry updating the prince on his plane at Vienna airport about the talks, diplomats said.

Sunni Gulf monarchies are wary of any US rapprochement with Shiite Iran. Better ties with Tehran could however lead to cooperation with the US in other areas, not least tackling Islamic State militants in Syria and Iraq.

Kerry on Saturday spoke to Israel's Prime Minister Benjamin Netanyahu by phone. The Jewish state, widely thought to have nuclear weapons itself, has not ruled out bombing Iran's nuclear facilities.

"We're working hard," Kerry said Saturday in Vienna. "And we hope we're making careful progress, but we have big gaps, we still have some serious gaps, which we're working to close."

Diplomats on both sides say that the two sides remain far apart on the two crucial points of contention: uranium enrichment and sanctions relief.

Enriching uranium renders it suitable for peaceful purposes like nuclear power but also at high purities for the fissile core of a nuclear weapon.

Tehran wants to massively ramp up the number of enrichment centrifuges -- in order, it says, to make fuel for future reactors -- while the West wants them dramatically reduced.

Iran wants painful UN and Western sanctions that have strangled its vital oil exports lifted, but the powers want to stagger any relief over a long period of time to ensure Iranian compliance with any deal.