World powers were set Tuesday to miss yet another deadline to nail down an elusive nuclear deal ending a 13-year standoff with Iran, despite hours of difficult top-level negotiations.

In a sign of how complex the negotiations have become, foreign ministers met deep into the night Monday grappling with the toughest remaining issues which have so far thwarted a deal to curtail Iran's nuclear programme.

In what has become a high-stakes game of diplomatic poker, the ministers met twice Monday with the Iranian delegation led by Foreign Minister Mohammad Javad Zarif for a total of almost three hours.

And even though he was seen smiling broadly when cameras were allowed a quick glimpse of the talks, there was little immediate sign of an end to the deadlock.

After busting through an initial June 30 deadline, the latest round of negotiations looked set to power beyond Tuesday's new target date.

Asked whether Tuesday's deadline may slip, White House spokesman Josh Earnest said: "I would say that it's certainly possible."

State Department spokesman John Kirby told reporters in Washington that Tuesday was "not a deadline. It was an extension of basically seven days of the parameters" of an April 2 framework accord struck in Lausanne.

But he refused to be drawn on what might happen on Tuesday, insisting: "Everybody is still I think rowing on the oars here to try to get a deal done, but it's got to be the right deal."

An Iranian official earlier made it clear that "July 7, July 8, we do not consider these dates as those dates we have to finish our job."

"Even if we pass July 9, that will not be the end of the world," the Iranian said, asking not to be identified.

The P5+1 group -- Britain, China, France, Germany, Russia and the United States -- are seeking to nail down an accord to put a nuclear bomb out of Iran's reach.

But despite progress on a series of complicated annexes, the negotiations have bogged down on how to ease a web of sanctions against Iran, probing allegations that in the past Tehran sought to develop nuclear arms, and ensuring Iran can continue to have a modest, peaceful nuclear programme.

- Anger at arms embargo -

In what appeared to be a new spanner in the works, the Iranian official said his country also wanted to ensure there was no renewal of a UN "arms embargo" enshrined within the new accord.

"There is no evidence that the arms embargo has any relation with the nuclear issue," the official said.

"The arms embargo should not be part of" the deal under negotiation, he said.

A fact sheet put out by the State Department after the April 2 framework accord said that once any deal is reached there would be a new UN Security Council resolution to extend "important restrictions on conventional arms and ballistic missiles".

Western officials are clearly balking at any notion of allowing Iran to buy conventional weapons, at a time when it is accused of fomenting unrest in the Middle East through alleged proxies such as Hezbollah militants.

"We are not there yet... We should not underestimate that important questions remain unresolved. There will not be an agreement at any price," a source within the German delegation said.

"If there is no movement in decisive areas a failure is not ruled out."

Officials from the UN's atomic watchdog meanwhile held "intense discussions" in Tehran on Monday about the possible military dimensions of the country's nuclear programme, an Iranian official said.

There was hope that a long-stalled probe into allegations that Iran, before 2003 and possibly after, sought to develop nuclear arms could finally be concluded after Yukiya Amano, the head of the International Atomic Energy Agency (IAEA) travelled to Iran last week.