European states will announce a multimillion-euro credit line to ease trade between the EU and Iran in a last-minute and probably doomed effort to persuade Tehran not to take the first step to quit the 2015 nuclear deal.

Iran’s withdrawal is almost certain to deepen the Gulf crisis, and prompt US demands for Europe states – primarily Britain, France and Germany – to quit the deal and join Washington in imposing sanctions on Tehran.

The relatively modest credit line will be announced in Vienna at a meeting of the joint commission for the Iran deal – the grouping of all the remaining parties to the deal including Europe, Iran, Russia and China.

The US pulled out of the deal, known as the Joint Comprehensive Plan of Action (JCPOA), a year ago. It imposed harsh economic sanctions on Iran that have plunged the country’s currency into a nosedive and created mass unemployment.

The credit line is expected to be announced as early as Thursday – the same day Iran will break part of the nuclear deal by surpassing the 300kg limit on its low-enriched uranium stockpile.

Iran is due to take further more serious steps to breach the deal on 7 July, when it has pledged to increase uranium enrichment purity levels over the 3.67% limit set in the deal. Officials said this could be a symbolic increase to 3.68%, but if the levels continue to rise Europeans would be concerned that the breakout time – the period it would take Iran to enrich enough uranium for a nuclear bomb – could fall below a year.

Iran will argue that its long-signalled moves to breach the limit do not breach the JCPOA, claiming it is entitled to take reversible steps to suspend parts of the deal if another signatory to the deal has failed to keep a commitment, notably the undertaking to boost trade between the EU and Iran.

The EU is likely to counter that Europe is using its best endeavours to boost trade, but has no obligation under the deal to take steps to compensate for the effect of threatened US sanctions on any firm that trades with Iran. The US sanctions regime – including the removal of all Iranian oil export waivers – has been much more draconian than Europe or Iran foresaw.

The EU will argue that the credit line should be seen by Tehran as a signal of intent to launch a trading mechanism focused initially on humanitarian goods that will allow companies to trade with minimum access to the banking system.

The EU is still not in a position to announce any specific transactions under the mechanism, but officials have repeatedly said that some deals will be announced in the next few days. The EU may not give specific transactions publicity for fear that it will alert the US Treasury to impose sanctions.

Iran has said that announcement of a credit line – briefed to the Iranians in lengthy technical talks in Tehran last week – hardly represents the seriousness of intent it requires to stay inside the JCPOA.

The idea of a trading mechanism was first announced in January, but it has proved a huge technical and legal task of international co-ordination for Europe to construct a scheme in which even mid-level European banks are willing to participate.

European officials recognise that the crisis could spiral out of control in the coming weeks, with the International Atomic Energy Agency declaring Iran is in breach of the deal in a matter of days. At the same time France, Germany and Italy are raising concerns about Iran’s ballistic missile programme, saying it is designed to be capable of delivering a nuclear payload.

European leaders will use the G20 summit of world leaders in Japan starting on Friday to press the US to de-escalate the crisis, and to clarify the precise demands it is making of Tehran. Russia and China are also likely to urge the US to back off. Japan as summit host has put itself forward as a mediator.

But Hassan Rouhani, the Iranian president, says he is not willing to negotiate while his country is under sanctions.