Iran and major powers will seek Monday to put the finishing touches to a historic nuclear deal, with both sides saying an accord ending a 13-year standoff lies tantalisingly within reach.

The talks, the latest set of which have dragged on for more than two weeks, aim to nail down an agreement curbing Iran's nuclear activities to make it extremely difficult for Tehran -- which denies any such goal -- to develop the atomic bomb.

There had been optimism that a deal would be clinched over the weekend, but finalising a framework accord struck in April has proved difficult, with talks stumbling on the exact timing of sanctions relief and Iran's desire to have a UN conventional arms embargo lifted.

"We have come a long way. We need to reach a peak and we're very close," President Hassan Rouhani said in Iran Sunday, quoted by the ISNA news agency on a 16th day of talks in Vienna.

"We are so close that if you look down from below you feel as if we have got there, but when you do get there you know there are still some steps to take."

A German diplomatic source warned the talks "could yet fail".

"But we really are nearly there. The decisive moment has arrived. If Tehran is ready to take the final steps then things can go very quickly," the source said late Sunday.

Such an accord, if it can be agreed, approved and upheld by all concerned, would draw a line under 13 years of failed diplomacy and threats of military action.

In return Iran will be granted staggered relief from painful sanctions, although the six powers -- the so-called P5+1 of the United States, Britain, France, Germany, Russia and China -- insist on the option of reimposing the restrictions if Tehran breaches the agreement.

- 'The final phase' -

Foreign ministers from all seven countries -- with the possible exception of China -- were expected to be present in Vienna on Monday.

"I hope we are finally entering the final phase of these marathon negotiations. I believe it," French Foreign Minister Laurent Fabius, who cancelled a trip to Africa to stay at the talks, said Sunday.

The current diplomatic effort dates back to Rouhani's coming to power in 2013.

He sought a rapprochement with the West and an end to his country's diplomatic and economic isolation.

The prospect of a thawing of relations between Iran and the United States unsettles many in the Middle East, however, not least Tehran's rivals Saudi Arabia and other Gulf monarchies.

Israel, widely assumed to have nuclear weapons itself, is also deeply concerned, complaining that the mooted deal will fail to stop its arch foe getting the bomb.

Russian Foreign Minister Sergei Lavrov returned to the Austrian capital on Sunday, joining US Secretary of State John Kerry, Iran's Mohammad Javad Zarif, Fabius, Germany's Frank-Walter Steinmeier and EU foreign policy chief Federica Mogherini.

British Foreign Secretary Philip Hammond, who left Vienna on Sunday afternoon, was expected back on Monday morning. Chinese Foreign Minister Wang Yi arrived in Vienna on Sunday, the country's official Xinhua news agency reported.

Several deadlines have been missed over the past more than two weeks of talks, although diplomats insist that the technical aspects of what will be a highly complex agreement are as good as sewn up.

On Monday the terms of a 2013 interim deal freezing parts of Iran's nuclear programme in return for minor sanctions relief expire, although this has been extended several times already.

The deal, if it can be sealed, will however prove a "hard sell" in the US Congress, top Republican and Senate Majority Leader Mitch McConnell said in an interview broadcast Sunday.

"It appears as if the administration's approach to this was to reach whatever agreement the Iranians are willing to enter into," he told Fox News Sunday.

'Decision time' nears in Iran nuclear talks
Vienna (AFP) July 12, 2015 - World powers talked deep into the night Saturday seeking to break a deadlock in marathon negotiations with Iran and strike a historic deal to rein in Tehran's nuclear ambitions.

A new deadline for an agreement is looming on Monday, and both sides are asking how much longer busy ministers can clear their schedules to hunker down in Vienna as they seek to end a 13-year standoff with the Islamic republic.

"Everything is on the table. It's now time to decide," French Foreign Minister Laurent Fabius warned on the 15th day of the tortuous talks.

Iran and the so-called P5+1 group, including Britain, China, France, Germany, Russia and the United States, are seeking to curtail Tehran's ability to develop nuclear weapons in exchange for relief from painful sanctions.

Three previous deadlines in this round of talks -- June 30, July 7 and July 9 -- have already been missed and officials at the negotiations said sanctions and an arms embargo remain key sticking points.

"Ninety-eight percent of the text is finished," said a source close to the discussions after a flurry of bilateral and multilateral talks throughout Saturday.

Two or three important questions remained, however, including how long any agreement made in Vienna should last, and over the lifting of international sanctions on Iran such as a UN arms embargo, the source added.

"Now there needs to be a political decision. And if that is taken things could quickly" progress, the person said.

US Secretary of State John Kerry tweeted that there were still "difficult issues to resolve" after emerging from almost 90 minutes of talks with his Iranian counterpart Mohammad Javad Zarif and EU foreign policy chief Federica Mogherini.

The three met again later in the evening, in between two meetings of the P5+1 group, which ended just before midnight.

Kerry had spoken earlier by telephone with Russian Foreign Minister Sergei Lavrov, but it remained unclear when Lavrov would return to the Austrian capital.

Under the parameters of a framework deal reached in Lausanne in April, Iran is to slash the number of its centrifuges from more than 19,000 to just over 6,000 and cut its stocks of enriched uranium, which can be used to make a bomb, from more than seven tonnes to about 350 kilos (770 pounds).

The aim is to ensure that it would take Iran at least a year -- from an estimated two to three months currently -- to acquire enough fissile material to build a bomb.

But for weeks experts have been wrangling over exactly how to implement the Lausanne guidelines.

- 'Arrogance' -

The negotiations have also stumbled over demands that UN nuclear inspectors have access to military sites, to investigate suspicions Iran sought to develop nuclear weapons in the past, allegations Tehran categorically denies.

The terms of an interim accord agreed in November 2013 have been extended until Monday to give the negotiators more time to strike an accord.

In Tehran, Iran's supreme leader Ayatollah Ali Khamenei said the battle against the "arrogance" of arch-foe the United States would continue.

"The US is perfect instance of Arrogance. Prepare yourselves for more fight against Arrogance," his English-language Twitter account quoted him as saying in a meeting with university students.

The toughest problems in the Iranian nuclear talks have been left to last, including a mechanism for lifting interlocking EU, US and UN sanctions.

A new headache has emerged in recent days, with the Iranian delegation insisting that a UN arms embargo be lifted once a deal is reached.

Expert Kelsey Davenport, from the Washington-based Arms Control Association think-tank, warned: "Now is not the time for brinksmanship or a hardening of positions."

"This is an historic moment and there could be serious repercussions if negotiators fail to seize this opportunity to get a good deal," she told AFP.