The last act of high-stakes Iran nuclear talks began yesterday with foreign ministers seeking to resolve a few remaining issues blocking a historic accord on the eve of a final deadline.

With ministers back in Vienna on a 10th day of talks, both sides warned however that success in their quest to nail down an agreement ending a 13-year-old standoff was far from guaranteed.

On Sunday, US Secretary of State John Kerry, in Vienna since July 27 facing off against his Iranian counterpart Mohammad Javad Zarif, said "it is now time" to seal the deal.

"While I completely agree... that we have never been closer, at this point this negotiation could go either way,” he said, with talks "not yet where we need to be on several of the most diffi-cultissues".

The point was rammed home by France's Foreign Minister Laurent Fabius, Germany's Frank-Walter

Steinmeier and EU foreign policy chief Federica Mogherini as they returned to Vienna late Sunday.

“The main question is to know whether the Iranians will accept making clear commitments on what until now has not been clarified," Fabius said, adding that "all the cards" were "now on the table".

Yesterday, an Iranian official said that Tehran has "made a number of concessions and “shown a good amount of flexibility" and that “only a few items., need to be tackled by the ministers".

The official, asking not to be identified, said however that “there are tough issues ... which were not resolved at expert level or the deputy minister level, those are not easy issues."

Also present were Russian Foreign Minister Sergei Lavrov and China's Wang Yi, who yesterday said that a "comprehensive agreement is within reach."

"What is important is that today and tomorrow all parties, especially the United States and Iran, should

make their final decisions as quickly as possible, " Wang said.

The deal, capping almost two years of rollercoaster talks following the 2013 election of Iranian President Hassan Rouhani, would build on a framework accord reached in April in Switzerland.

A final deal could also put Iran and the United States on the road to normalised relations - much to the alarm of Iran's regional rivals - after 35 years of enmity and mistrust.

Signs have emerged that some of the outstanding issues have been resolved by teams of experts, who are now waiting for the ministers to sign off on their months of behind-the-scenes work.

But the same tough issues that remain have bedevilled the talks since the start.

These include investigating allegations that in the past Iran sought nuclear weapons, finding a mechanism to lift the sanctions, and ensuring Iran can continue to have a modest, peaceful nuclear programme.