Top diplomats worked past a midnight deadline, seeking to agree the outlines of a nuclear deal with Iran, insisting "enough progress" had been made to continue the marathon talks.

"We've made enough progress in the last days to merit staying until Wednesday. There are several difficult issues still remaining," US State Department spokeswoman Marie Harf said.

The announcement came late on a sixth day of talks in Switzerland aimed at laying the groundwork for a deal that world powers hope will prevent Iran developing nuclear weapons under the guise of its civilian program.

The stakes are high, with fears that failure to reach a deal may set the United States and Israel on a road to military action to thwart Iran's nuclear drive, which Tehran says is purely peaceful.

Senior Iranian negotiator Hamid Baidinejad also hinted at progress, saying "many of the issues linked to the sanctions have been resolved".

Iran has been calling for an immediate lifting of the sanctions, while the six powers are only prepared to suspend sanctions to be able to put them back into place if Tehran violates the deal.

An army of technical and sanctions experts would continue plugging away "for (the) next hours. All parties (are) working hard and (are) committed to finding a solution," a Western diplomat said.

Under a deal to be finalised by June 30, the powers want Iran to scale back its nuclear program to give the world ample notice of any dash to make the bomb by extending the so-called "breakout" time.

In return, the Islamic republic is demanding the lifting of sanctions that have strangled its economy.

But the question is how much detail will be in the framework accord that Iran and the six powers - the US, China, Russia, Britain, France and Germany - want to nail down.