Iran and major powers strive to finally nail down an elusive nuclear accord ahead of a Monday deadline, with China calling for "no more delays" in the marathon talks.

Raising hopes of an imminent breakthrough, Iran said that President Hassan Rouhani would address the nation on state television at 1730 GMT.

After more than two weeks of intense political haggling in Vienna aimed at ensuring Iran does not get a nuclear bomb, diplomats said they were in the final stage of negotiations.

Chinese Foreign Minister Wang Yi said the time had come to wrap up the talks, now in their 17th straight day.

"China believes that no agreement could be perfect and conditions are already in place for us to reach a good agreement," he told reporters as he arrived for discussions in the Austrian capital, speaking through an interpreter.

Foreign ministers from the so-called P5+1 group of major powers were gathering "to bring the negotiation to its conclusion," he added.

'Political will required'

"We believe that there cannot and should not be further delay."

Abbas Araghchi, Iran's lead negotiator and deputy foreign minister, told Iranian media that he was hopeful for a deal on Monday or Tuesday.

"We are hopeful. If we can solve the problems and if (solutions) are accepted by the parties, God willing... I cannot make any promises for this evening or tomorrow evening, but there is hope," he said.

The six major powers -- the United States, Britain, France, Germany, Russia and China -- want Iran to scale down its atomic activities in return for an easing of crippling sanctions.

They have already missed several deadlines in the highly complex discussions in Vienna, but diplomats said they were not considering extending talks beyond the current deadline of Monday.

"No one is thinking of another extension. Everyone working hard to get to yes today, but political will still required," Iranian diplomat Alireza Miryousefi said on Twitter.

A source close to Iranian negotiators told AFP there were still "some important issues" to be resolved.

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.

Such an accord, if it can be agreed, approved and implemented properly -- which is also no small challenge -- would draw a line under 13 years of failed diplomacy and threats of military action.

'The final phase'

In return Iran will be granted staggered relief from painful sanctions, although the six powers insist on the option of reimposing the restrictions if Tehran breaches the agreement.

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 proposed deal will fail to stop its arch foe getting the bomb.

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

British Foreign Secretary Philip Hammond was expected back early on Monday afternoon.

Kerry huddled with the rest of the P5+1 for fresh talks on Monday morning. When asked whether the deadline might be pushed back again, he did not reply.

Speaking in Brussels, French President Francois Hollande said the negotiations were picking up pace.

"We are not necessarily very far" from an agreement but that "does not mean we are there yet," he said.

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.

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

But Kelsey Davenport, Arms Control Association analyst, said she does not expect the Republicans to be able to scupper what would be President Barack Obama's biggest foreign policy achievement.

"If the administration presents a good deal that blocks Iran's pathways to nuclear weapons and puts in place intrusive monitoring it should garner enough support from Congress to ensure implementation," Davenport told AFP.

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