Meeting of Joint Commission of the Joint Comprehensive Plan of Action in Vienna. (AFP)

Diplomats from the remaining parties to the 2015 Iran nuclear accord are meeting in Vienna to discuss ways to stop the deal collapsing amid tension in the Persian Gulf.

While US President Donald Trump said on Friday there was "no rush" to solve the standoff with Iran, the landmark 2015 Joint Comprehensive Plan of Action (JCPOA) looks perilously close to unravelling.

The "joint commission" meeting began just after 12:00 (10:00 GMT) in the presence of diplomats from the accord's remaining parties -- Iran, the EU, Germany, France, the UK, Russia and China -- including Iranian Deputy Foreign Minster Abbas Araghchi.

It comes as Iran stands on the brink of exceeding one of the limits on stockpiles of nuclear material foreseen under the JCPOA.

Iran says it no longer feels bound by certain limits in the deal due to the crippling sanctions re-imposed by the United States after it unilaterally pulled out of the accord in May 2018.

Originally Iran had said it would exceed the agreed 300-kilogram (660-pound) reserve of enriched uranium on Thursday but diplomatic sources said that the limit had not been passed.

However, an Iranian official said Thursday said the fact the date had been missed was a "technical matter" and that Iran was still not ruling out breaching the limit.

He said that Iran was currently 2.8kg under the 300kg ceiling.

Iran has also said that as of July 7 it will start enriching uranium above the agreed purification level of 3.67 percent.

Weapons-grade levels are about 90 percent.

However, the Iranian official stressed that even if both limits were surpassed, this could be swiftly reversible "within half an hour" if there were progress on facilitating Iran's oil exports, which have been aggressively targeted by the US.

It is thought that the remaining European parties to the deal will try to step up funding to the INSTEX mechanism which is meant to protect trade with Iran from US sanctions.

But it remains to be seen whether the Iranians will judge this to be sufficient progress, particularly if it does not help Iran's vital oil sector.

Speaking in Osaka ahead of the G20 summit of world leaders, US President Donald Trump struck an emollient tone on resolving the Iran crisis, saying: "They can take their time. There is absolutely no time pressure."

On Wednesday, Trump had said any conflict "wouldn't last very long," an assertion quickly rejected by Iran's foreign minister, as well as French President Emmanuel Macron.

Under the landmark deal signed with world powers in 2015, Iran pledged to reduce its nuclear capacities for several years and to allow international inspectors into the country to monitor its activities in return for relief from international sanctions.