An Iranian source voices Tehran’s readiness to extend by up to a year if no real progress toward a deal is achieved later Sunday.

Top negotiators of Iran, the US and the EU are meeting in another trilateral meeting ahead of the Monday nuclear deadline, with an Iranian source voices Tehran’s readiness to extend by up to a year if no real progress toward a deal is achieved later Sunday.

Iranian Foreign Minister Mohammad Javad Zarif, his US counterpart John Kerry and EU nuclear negotiator Catherine Ashton are holding marathon meeting at Vienna’s Coburg Hotel on Sunday before noon.

Meanwhile, Iranian Deputy Foreign Ministers Abbas Araqchi and Majid Takht-e-Ravanchi are holding talks with delegates from China and the US as well as Russian Deputy Foreign Minister Sergei Ryabkov and Ashton’s deputy Helga Schmid.

On Sunday, Russian Foreign Minister Sergei Lavrov will also head to the Austrian capital to join the nuclear negotiations.

In the meantime on Sunday, An Iranian source told AFP that Iran is open to having the nuclear negotiations extended by six months or a year if no real progress toward an agreement is achieved later Sunday.

"We are still focused on agreeing to a kind of political agreement" which would not be written but which would allow for negotiators to fine-tune technical aspects of the agreement later, the source said.

"But if between now and this afternoon or this evening we don't get there, the solution is we consider an extension of the Geneva accord," he said.

"That could be for a period of six months or a year. We must absolutely avoid a climate of confrontation with escalation from one side and the other," the source said.

The parties have been holding talks in Vienna over the past six days to hammer out a comprehensive deal on Tehran’s nuclear energy program. Diplomats say the two sides have made progress but big gaps still remain.

For his part, Kerry said on Saturday: "We're working hard, and we hope we're making careful progress, but we have big gaps, we still have some serious gaps, which we're working to close."

US Secretary of State who on Friday postponed a trip to Paris to remain in Vienna for the talks, met Iranian Zarif on Saturday afternoon, their fourth meeting in three days.

German Foreign Minister Frank-Walter Steinmeier, also in the Austrian capital, called this final weekend of talks, after months of negotiations, a "moment of truth".