A nuclear deal with Iran is possible at the next round of talks in Geneva, a US official said Friday, but warned tough issues still had to be hammered out.

"We are going to work very hard next week. I don't know if we'll reach an agreement. I think it is quite possible that we can. But there's still tough issues to negotiate," the senior administration official told reporters.

The official also renewed pleas to skeptical US lawmakers not to slap more sanctions on Iran in the mistaken belief it would force the Islamic republic "to the point of capitulation" and the dismantling of all its nuclear program.

"I personally don't believe surrender would come any time soon, it is a culture of resistance," the official, warning that if there were new sanctions Iran "would move forward with its nuclear program ... and we would find ourselves with no other option but a military one."

Iranian Foreign Minister Mohammed Javad Zarif is to join political directors from the six powers negotiating a freeze to Iran's suspect nuclear program for a new round of talks in the Swiss city from Wednesday.

The last session failed on Saturday to seal a first step deal which would halt Iran's program in return for relief from crippling sanctions while all sides negotiate a final deal over the next six months.

At the end of marathon three-day talks in Geneva the so-called P5+1 group had presented a "stronger" and "improved" draft deal to Iran which had "greater clarity" on a number of issues.

But the negotiations ended early Saturday morning "because I think the parties, particularly Iran, felt they needed to go back and look at this document which was quite tough, consider it and come back to negotiations," the US official said.

But the official downplayed reports of divisions among the P5+1, which is comprised of the five nuclear powers Britain, France, China, Russia and the United States, as well as Germany, saying it was normal that each capital had weighed in with changes to the document.

And while officials stuck to an agreement not to reveal any details of the proposal, it is understood it would give Iran access to "a very small fraction" of its assets frozen in bank accounts around the world.

Another senior administration official said as much as $100 billion in revenues from Iranian oil sales was stuck in global banking accounts.

Iranian oil exports have fallen to around a million barrels a day, "dramatically down from an average of about two and a half million barrels a day in 2011," he said.

"These declining exports are costing Iran up to $5 billion a month, and have cost Iran along with our other sanctions about $120 billion," he said.

"So the relief that we are considering as part of the initial phase would be limited, temporary, targeted and reversible," he said, insisting the core structure of sanctions would stay in place.

Shooting down reports that the US and P5+1 were planning a major, valuable package of sanctions relief, he said "it would come nowhere near helping Iran escaping the hole that we've put them in."

Officials from the administration of President Barack Obama have been leading what they described as "hard" discussions with US lawmakers seeking to head off a new round of American sanctions which they fear could scupper the delicate negotiations.

"Further sanctions as this moment, not for all time perhaps because we don't know what Iran will do, but at this moment, further sanctions threaten the good faith of that negotiation, not just with Iran but with our partners," the first administration official said.

All the P5+1 members had asked the official to convey to Congress their desire for a pause to any more sanctions.

"The P5+1 believes these are serious negotiations, they have a chance to be successful. For us to slap a new set of sanctions in the middle of it they see as bad faith with them."