Iran and major powers stood on the brink Tuesday of a historic deal aimed at ensuring Tehran does not acquire a nuclear bomb, with a final ministerial meeting called in Vienna.

The apparent breakthrough came on the 18th day of marathon talks between Tehran and the so-called P5+1 -- the United States, Russia, China, Britain, France and Germany.

"Final plenary of E3/EU+3 and Iran at 10h30 at the UN," EU spokeswoman Catherine Ray wrote on Twitter following growing signs that an agreement was within reach.

She said the meeting would be followed by a press conference.

US State Department spokeswoman Marie Harf tweeted the same schedule.

The deal is expected to sharply curb Iran's nuclear programme and impose strict UN inspections in order to make any drive to make nuclear weapons all but impossible and easily detectable.

In return, the web of UN and Western sanctions choking Iranian oil exports and the economy of the 78-million-strong country would be progressively lifted.

The diplomatic push began when Iranian President Hassan Rouhani came to power in 2013. In November that year an interim deal was agreed but two deadlines in 2014 for a lasting accord were missed.

Then in April, the parties scored a major breakthrough by agreeing the main outlines of an accord, aiming to finalise it by June 30, a deadline since pushed back twice.

Since April, legions of legal and technical experts have made great strides working out the nuts and bolts of how the highly ambitious and technical agreement will work.

The final hurdles included the exact timing and pace of sanctions relief and Iran's desire to have a UN arms embargo lifted.

The White House said on Monday that the marathon discussions in Vienna had "made genuine progress".

"But there continues to be some sticking points that remain unresolved," White House spokesman Josh Earnest told reporters in Washington.

He said the United States and its partners did not want to rush the final stages of the lengthy talks.

"Typically, some of the most difficult issues are the ones that get kicked to the end, and that's why the president is going to resist any effort to sort of fast-forward through the closing here," Earnest said.

- 'Conditions in place' -

Foreign ministers including US Secretary of State John Kerry huddled late into the night at Vienna's Palais Coburg.

China's Foreign Minister Wang Yi told reporters as he arrived in the Austrian capital on Monday that there should be "no more delays".

No deal could be "perfect" but "conditions are already in place for a good agreement," Wang said through an interpreter.

Zarif said on Monday he believed there should be no further extension to the talks.

"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.

Also present were Russian Foreign Minister Sergei Lavrov and his counterparts from Britain, France and Germany -- Philip Hammond, Laurent Fabius and Frank-Walter Steinmeier.

- Regional concerns -

If a deal can be sewn up, the prospect of a subsequent possible thawing of US-Iranian relations unsettles many in the Middle East, however.

These include Shiite Iran's Sunni-ruled rivals Saudi Arabia and other Gulf monarchies who see Tehran as a destabilising influence in the region.

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.

Many in the United States, particularly among US President Barack Obama's Republican opponents, agree that the mooted deal is too weak.

The agreement will prove a "hard sell" in the US Congress, which will have 60 days to chew over the accord, top Republican Mitch McConnell said in an interview broadcast Sunday.

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