Iran said some progress had been made at a meeting with world powers on its nuclear accord – but probably “still not enough” to keep the landmark 2015 deal alive.

“It was a step forward, but it is still not enough and not meeting Iran’s expectations,” said Iran’s deputy foreign minister, Abbas Araghchi, after the talks on Friday. “I don’t think the progress made today will be enough to stop our process – but the decision will be made in Tehran.”

Tehran has threatened to withdraw from key commitments in the deal, probably on 7 July, a move that might force European powers to reluctantly join the US and quit the deal signed in 2015.

Araghchi said: “The decision to reduce our commitments has already been made in Iran and we continue on that process unless our expectations are met.”

But he also said a ministerial-level meeting would be held soon, probably giving Europe a further chance to save the deal from imminent collapse.

Q&A What is the Iran nuclear deal? Show In July 2015, Iran and a six-nation negotiating group reached a landmark agreement known as the Joint Comprehensive Plan of Action that ended a 12-year deadlock over Tehran’s nuclear programme. The deal, struck in Vienna after nearly two years of intensive talks, limited the Iranian programme, to reassure the rest of the world that it cannot develop nuclear weapons, in return for sanctions relief.

At its core, the JCPOA is a straightforward bargain: Iran’s acceptance of strict limits on its nuclear programme in return for an escape from the sanctions that grew up around its economy over a decade prior to the accord. Under the deal, Iran unplugged two-thirds of its centrifuges, shipped out 98% of its enriched uranium and filled its plutonium production reactor with concrete. Tehran also accepted extensive monitoring by the International Atomic Energy Agency (IAEA), which has verified 10 times since the agreement, and as recently as February, that Tehran has complied with its terms. In return, all nuclear-related sanctions were lifted in January 2016, reconnecting Iran to global markets. The six major powers involved in the nuclear talks with Iran were in a group known as the P5+1: the UN security council’s five permanent members – China, France, Russia, the UK and the US – and Germany. The nuclear deal is also enshrined in a UN security council resolution that incorporated it into international law. The 15 members of the council at the time unanimously endorsed the agreement. On 8 May 2018, US president Donald Trump pulled his country out of the deal. Iran announced its partial withdrawal from the nuclear deal a year later. Saeed Kamali Dehghan, Iran correspondent

Such a collapse would probably lead the EU to join the US in imposing sanctions, which might bring a military response by Iran as it tries to find levers to force the west to loosen the vice on its economy.

At the talks, the three European powers – France, Germany and the UK – tried to assuage Iranian doubts that Europe was sticking to its side of the 2015 bargain by pressing ahead with the long-planned mechanism to facilitate trade between Iran and Europe without being hit by US sanctions.

The lead EU negotiator, Helga Schmid, hailed the discussions as constructive and said the first transactions had been completed through the mechanism, known as Instex.

The EU was also due to supply a modest €3m (£2.7m) credit line to kickstart the mechanism as a signal of good intent.

However, Araghchi said: “For Instex to be useful for Iran, Europeans need to buy oil or consider credit lines for this mechanism, otherwise Instex is not like they or us expect.”

The EU has always said Instex will be confined to easing trade in humanitarian goods, such as food and medicine – a form of trading the White House says is not subject to its sanctions regime.

Iran had billed the Vienna meeting as the last chance to salvage the deal.

The Chinese envoy, Fu Cong, said Beijing rejected the US policy of unilateral sanctions, suggesting China may be willing to defy the US threat.

But the US special representative on Iran, Brian Hook, insisted the US would put sanctions on all Iranian oil exports. “We will sanction any illicit purchases of Iranian crude oil. There are right now no oil waivers in place,” he said.

Hook said the US economic grip on Iran was intended to force Tehran to renegotiate the original nuclear deal, turning it into a binding international treaty that also covers wider issues including Iran’s ballistic missile programme, its regional interference and its treatment of dual national prisoners.

He also promised the US would go ahead with plans to impose sanctions on Iran’s chief diplomat, Javad Zarif. He said Iran’s goal was to bring the Middle East under the thrall of Tehran’s “Marxist theocratic” ideology.

Hook was in London to urge the International Maritime Organization (IMO), a UN body, to back his plan for a UN-endorsed maritime protection force, modelled on an established UN anti-piracy force off the coast of Somalia.

He said Iran, by its threats to Gulf shipping, was “courting a financial and environment disaster”, and was breaching all maritime rules by turning off its ship collision avoidance transponders in an attempt to smuggle oil out of Iran and avoid being tracked by the US.

“An outlaw regime is violating basic maritime law and now represents a maritime threat across the Gulf,” Hook said after meeting the IMO secretary general, Kitack Lim.

Hook went further than before by accusing Iran of planning to gain a stranglehold on all international oil-related shipping by controlling not just the Gulf of Oman, off the Iranian coast, but also the Red Sea shipping lanes through the Bab al-Mandab strait off Yemen that take oil into the Arabian Sea and on to Asia.

Q&A Why is the Gulf of Oman so important for shipping oil? Show The strait of Hormuz, which provides passage from the Gulf of Oman to the open sea, is the most important gateway for oil exports in the world. With Iran on its northern shore, and the UAE and Oman on its southern shore, the US Energy Information Administration (EIA) calls it the world’s worst 'chokepoint' In 2016, 18.5m barrels of crude oil were transported each day through the strait of Hormuz, compared with 16m through the strait of Malacca, which runs between the Indonesian island of Sumatra, Malaysia and Thailand, connecting the Indian Ocean with the South China Sea. 5m barrels of crude oil are transported annually through the next largest chokepoint, the Suez canal. Phillip Inman

His claim shows the extent to which the US regards defeat of the Iranian-backed Houthis in Yemen as integral to the wider defeat of Iran.

“If we do not prevent Iranians from laying down deep roots in Yemen, they will be in a position to threaten to close the strait of Hormuz and Bab al-Mandab,” Hook said.

“They are seeking to do in Yemen what they succeeded to do in Lebanon. They would like to ‘Lebanonise’ Yemen and become a power broker on the Saudi southern border and to be in a position to harass the Emiratis and Saudis, and to be another threat in their threat matrix to freedom of navigation. We are pushing back on Iran’s long game in Yemen.”

The sanctions, hailed by Hook as unprecedented, have so far cut Iranian oil exports to 400,000 barrels a day, levels that have put the Iranian government deep into debt.

“This is a clerical regime that wants to remake the Middle East in its image – and that would deeply destabilise the Middle East to have regimes following the same Marxist theocratic regime,” Hook said.

Describing the state of tension in the region, he said Iran’s goal “is to scare the global oil market to keep the price up and to keep the world on edge”.