Iran and major powers looked set Monday for another late night of talks on a nuclear deal to end a 13-year standoff, as they struggled to overcome the stubborn remaining obstacles.

"The work is not yet finished. We are working without respite," a source close to Iranian negotiators said. Asked if the talks would continue for an 18th day on Tuesday, he said: "This depends on our work tonight."

Another Iranian source said the chances of a deal on Monday were "low".

Over the weekend hopes had been raised that the end might be finally in sight and that ministers from Iran and the six powers might be able to nail down the agreement on Monday.

China's Foreign Minister Wang Yi, joining his counterparts at the Palais Coburg hotel in Vienna where the talks were being held, told reporters that there should be "no more delays".

No deal could be "perfect" but "conditions are already in place for a good agreement", Wang said through an interpreter, before going into talks with US Secretary of State John Kerry and other ministers.

Iranian Foreign Minister Mohammad Javad Zarif said he believed there should be no further extension to the talks -- but added that he would negotiate as long as needed.

"I always believe there shouldn't be any extension but we could work as long as necessary to finish this," Zarif said as he met Wang.

Also present were Russian Foreign Minister Sergei Lavrov and his counterparts from Britain, France and Germany -- Philip Hammond, Laurent Fabius and Frank-Walter Steinmeier.

- 'Political will required' -

The P5+1 group -- the United States, Russia, China, Britain, France and Germany -- want Iran to scale down its atomic activities in order to make any drive to make the atomic bomb all but impossible.

The current diplomatic effort began when Iranian President Hassan Rouhani came to power in 2013. In November that year an interim deal was agreed but two deadlines in 2014 for a lasting accord were missed.

Then in April, the parties scored a major breakthrough by agreeing the main outlines of an accord, aiming to finalise it by June 30, a deadline since pushed back twice.

The terms of the November 2013 interim deal under which Iran froze certain nuclear activities in return for minor sanctions relief were due to expire on Monday night, although they have been extended before.

Much of the technical work in what will be a highly complex accord is done, but the talks have stumbled on the exact timing of sanctions relief and Iran's desire to have a UN conventional arms embargo lifted.

Earlier Monday Iranian diplomat Alireza Miryousefi said on Twitter that "political will" was still needed. An Iranian source said "some important issues" remained to be resolved.

- Regional arms race? -

If a deal can be sewn up, the prospect of a thawing of US-Iranian relations unsettles many in the Middle East, however.

These include Shiite Iran's Sunni-ruled rivals Saudi Arabia and other Gulf monarchies who see Tehran as a destabilising influence in the region.

Israel, widely assumed to have nuclear weapons itself, is also deeply concerned, complaining that the proposed deal will fail to stop its arch foe getting the bomb.

"We are heading toward a bad deal, and in the period after it we will of course have to continue preparing to protect ourselves on our own," Israeli Defence Minister Moshe Yaalon said on Monday.

Many in the United States, particularly among US President Barack Obama's Republican opponents, agree that the mooted deal is too weak.

The agreement will prove a "hard sell" in the US Congress, which will have 60 days to chew over the accord, top Republican Mitch McConnell said in an interview broadcast Sunday.