Time runs out Monday for the biggest chance in years to resolve the Iranian nuclear standoff, as Tehran and world powers make a final push for a deal but with a risky extension looking likely.

The five permanent members of the UN Security Council and Germany (the P5+1) have been locked in talks with Iran for months, seeking to turn an interim deal that expires at midnight (2300 GMT) on Monday into a lasting accord.

Such an agreement, after a 12-year standoff, is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities, an ambition it hotly denies.

But a last-ditch diplomatic blitz in recent days involving US Secretary of State John Kerry and other foreign ministers to secure a deal appears to have failed to bridge the remaining major differences.

As a result, late Sunday a senior US State Department official said for the first time that the powers and Iran were now discussing putting more time on the clock.

The official said it was "only natural that just over 24 hours from the deadline we are discussing a range of options ... An extension is one of those options."

This came after US Secretary of State John Kerry met his Iranian counterpart Mohammad Javad Zarif for the sixth time since Thursday in an attempt to break the deadlock.

British Foreign Secretary Philip Hammond said however that the parties would still make a "big push tomorrow (Monday) morning to try and get this across the line".

"Of course if we're not able to do it, we'll then look at where we go from there," he said.

"We're still quite a long way apart and there are some very tough and complex issues to deal with".

Chinese Foreign Minister Wang Yi was expected in the Austrian capital early Monday, completing the line-up of all the six powers' foreign ministers.

This included Russian Foreign Minister Sergei Lavrov, a key player in the talks. Earlier in the week he said all the elements were in place for a deal with just "political will" missing.

- Gaps -

Diplomats on both sides say that despite some progress, 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.

"What a deal would do is take a big piece of business off the table and perhaps begin a long process in which the relationship not just between Iran and us but the relationship between Iran and the world, and the region, begins to change," US President Barack Obama in an ABC News interview aired Sunday.

- Extension -

In view of the difficulties -- and of the dangers posed by the alternative of a complete collapse -- many experts have long believed that the negotiators would put more time on the clock.

An Iranian source told AFP earlier Sunday, while stressing at that point that adding time was not yet on the table, that the extension "could be for a period of six months or a year."

Another extension -- as happened with an earlier deadline of July 20 -- however carries risks of its own,including possible fresh US sanctions that could lead Iran to walk away.

Pushing back the cut-off point will also fuel accusations from Israel, the Middle East's sole if undeclared nuclear-armed state, that its arch foe Iran is merely buying time to get closer to the bomb.

Arms Control Association analyst Kelsey Davenport told AFP that an extension of six months to a year "would not fly" with the other parties.

Any extension "will have to be very short because there are too many hardliners, particularly in Washington and Tehran, that want to sabotage this deal," she told AFP.