"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.
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.
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.
Another senior administration official said as much as USD 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.
