Top ministers were due to meet Monday seeking to conclude the nuclear deal with Iran, on the eve of the deadline set by top negotiators.

Top ministers were due to meet Monday seeking to conclude the nuclear deal with Iran, on the eve of the deadline set by top negotiators.

US Secretary of State John Kerry, who met four times on Sunday with his Iranian counterpart Mohammad Javad Zarif, acknowledged that at the eleventh hour the talks still "could go either way".

"All the cards are on the table," added French Foreign minister Laurent Fabius, as he arrived back in Vienna.

"If all sides were prepared to make hard choices, then we could get an agreement this week. But if they are not made, we will not," Kerry warned, adding that if there was "absolute intransigence" the US would walk away.

"The main question is to know whether the Iranians will accept making clear commitments on what until now has not been clarified," Fabius added.

The global powers (Britain, China, France, Germany, Russia and the United States) are trying to pin down a deal guaranteeing that a nuclear bomb is out of Iran's reach in return for lifting a web of sanctions against the Islamic republic.

"The time is now... We are very close," said the EU's foreign policy chief Federica Mogherini, adding the atmosphere was "constructive, positive".

For his part, Zarif said in an English YouTube message that an accord could "open new horizons to address important common challenges."