Iran and major powers scrambled to finally nail down an elusive nuclear accord ahead of a Monday deadline, with China calling for "no more delays" in the marathon talks.
After more than two weeks of intense political haggling in Vienna aimed at ensuring Iran does not get a nuclear bomb, diplomats said an agreement was tantalisingly close.
Chinese Foreign Minister Wang Yi said the time had come to wrap up the talks, now in their 17th straight day.
No deal could be "perfect" but "conditions are already in place for 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 -- the United States, Britain, France, Germany, Russia and China -- were gathering "to bring the negotiation to its conclusion," he added.
"We believe that there cannot and should not be further delay."
Iranian Foreign Minister Mohammad Javad Zarif said he believed there should be no further extension to the talks but he would negotiate as long as needed.
- 'Political will required' -
"I always believe there shouldn't be any extension but we could work as long as necessary to finish this," Zarif said as he met Wang.
Iran's President Hassan Rouhani will address the nation about the nuclear talks on state television on Monday night, a media official told AFP in Tehran.
The official IRNA news agency said Rouhani would speak when the nuclear talks have concluded, but it did not give a time.
The six major powers 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 were hopeful that this time would be different.
"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.
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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.
- Regional arms race? -
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 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.
"We are heading toward a bad deal, and in the period after it we will of course have to continue preparing to protect ourselves on our own," Israeli Defence Minister Moshe Yaalon said on Monday.
"It naturally contains implications on other states which perceive this situation as a threat, neighbouring states that are talking about their need to be armed, which could start a regional nuclear armament race," he said.
US Secretary of State John 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.
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.