World powers and Iran will make a "last push" for a nuclear deal before a Monday midnight deadline even though the parties are far apart, British Foreign Secretary Philip Hammond has said.

"At the moment we're focused on the last push, a big push tomorrow morning to try and get this across the line," Hammond told reporters on Sunday in Vienna when asked if an extension of the deadline was being considered.

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

Philip Hammond, UK Foreign Secretary

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

A senior US official acknowledged for the first time on Sunday that Iran and world powers were now discussing a possible extension of their deadline for a deal amid wide gaps in the talks. The West wants Iran to scale back its nuclear ambitions in exchange for lifting economic sanctions.

Hammond said the parties would still try to bridge the gaps even though it was not clear whether that could be.

"We're all focused on trying to get to a deal but I wouldn't want to give any false hopes here. We're still quite a long way apart and there are some very tough and complex issues to deal with but we're all focused right now on trying to get that deal done," he said.

Hammond said he was consulting his counterparts from Germany and France on Sunday evening on how to "manage the meetings" on Monday with the Iranian negotiators.

Britain, the US, Russia, China, France and Germany have been locked in talks with Iran for months to turn an interim deal that expires on Monday into a lasting accord.

Pro-nuclear rally

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

Clerics and students protested against the sanctions in front of the Tehran Research Nuclear Reactor [EPA]

US Secretary of State John Kerry had earlier said in Vienna that "serious gaps" remained between them and the Iranians.

"We're working hard," he said.

Meanwhile, pro-nuclear clerics and students took to the streets in the Iranian capital to demand that their government should not to cave in to Western pressure.

Around 300 demonstrators chanting "death to America and Israel" gathered near Tehran's nuclear reactor.

Iranian officials have refused to reduce the volume of uranium they are capable of enriching, a stand Western officials say is unacceptable as this would potentially allow Iran to amass enough fissile material for an atomic bomb in a short time.

A second sticking point is the pace and sequencing of sanctions relief. Iran wants them terminated swiftly, not suspended and gradually scrapped, depending on the degree of Iranian compliance with the deal terms, as the West wants.