VIENNA (AFP) -- The last act of high-stakes Iran nuclear talks began Monday 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 difficult issues".

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".

On Monday, 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 said Monday 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 US, Russia, China, Britain, France and Germany want Iran to sharply curb its nuclear programme to make any push to acquire the atomic bomb all but impossible, in return for sanctions relief.

Iran denies wanting nuclear weapons, saying its activities are purely for peaceful purposes such as generating electricity and treating cancer patients.

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.

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

He denounced the "growing menace of violent extremism and outright barbarism" in an reference to the Islamic State militant group -- a common enemy for Tehran and Washington.

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.

On Sunday, senior officials from the International Atomic Energy Agency

flew to Tehran for talks, following on the heels of the UN watchdog's chief Yukiya Amano on Thursday.

Amano said on his return that the agency could complete by the end of the year a stalled probe into allegations that before 2003, and possibly since, Iran had sought to develop nuclear arms.

"I think it would be very difficult to imagine Secretary Kerry at this point walking away, this close to the finish line," Iran expert Suzanne Maloney from the Brookings Institution told AFP.

"I just don't think there's any real likelihood that this collapses."