“It was a step forward, but it is still not enough and not meeting Iran’s expectations,” said Iran’s deputy foreign minister, Abbas Araqchi, 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.

Araqchi 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.

Such a collapse would probably lead the EU to join the U.S. 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, Araqchi 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.