Iran and world powers looked likely Monday to miss a midnight deadline to agree a long-awaited nuclear deal, with a Western diplomat saying they would agree an extension and meet again next month.
"Given progress made this weekend, talks headed to likely extension with experts and negotiating teams reconvening in December at a yet to be determined location," the diplomat said.
As the foreign ministers of Iran and the five permanent members of the UN Security Council plus Germany, the so-called P5+1, went into a meeting in Vienna, it was unclear how long the extension could be.
In the best chance in years to resolve the long-running standoff over Iran's nuclear programme, the P5+1 have been locked in talks with Iran for months, seeking to turn an interim deal that expires at midnight (2300 GMT) 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 strongly denies.
It could see painful sanctions on Iran lifted, silence talk of war and represent a much-needed success for both US President Barack Obama and his Iranian counterpart Hassan Rouhani.
A deal could begin a 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," Obama said in an ABC News interview Sunday.
But a last-ditch diplomatic blitz in Vienna in recent days involving US Secretary of State John Kerry and other foreign ministers appeared to have failed to bridge the remaining major differences.
As a result, Iranian and US officials said late Sunday they had started talking about plan B -- the option of putting more time on the diplomatic clock.
"We have discussed an extentions but there is nothing detailed yet," an Iranian source told AFP.
This came after Kerry met his Iranian counterpart Mohammad Javad Zarif for the seventh time since Thursday but again apparently failed to break the deadlock.
Chinese Foreign Minister Wang Yi arrived in the Austrian capital early Monday, completing the line-up of all foreign ministers from the six powers.
They included Russian Foreign Minister Sergei Lavrov, a key player in the negotiations whose country has built a nuclear plant in Iran and is planning more.
President Vladimir Putin was due to talk to Rouhani by phone later Monday, ITAR-TASS reported.
Diplomats on both sides say that despite some progress, the two sides remain far apart on the two crucial points: 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 a fleet of power reactors that it is however yet to build.
The West wants them dramatically reduced, a move which together with more stringent UN inspections and an export of Iran's uranium stocks, would make any attempt to make the bomb all but impossible.
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 to ensure Tehran complies with any deal.
In view of the difficulties, many experts have long believed that the negotiators would put more time on the clock, although how this might work is unclear.
The terms of the 2013 interim deal -- under which Iran froze some activities and won limited sanctions relief -- could be rolled over for a certain period of time.
An Iranian source told AFP on Sunday that the extension could be "six months or a year".
Alternatively there could be a new interim deal or "political framework", adding certain measures but leaving sanctions and enrichment until later. Or there could just be an agreement to keep talking.
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.
It will also fuel accusations from Israel, the Middle East's sole if undeclared nuclear-armed state, that its archfoe is merely buying time to get closer to the bomb.
Arms Control Association analyst Kelsey Davenport told AFP that 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".