In a letter to the 15-member body, seen by Reuters, the three European parties to the nuclear deal and long-time US allies said any decision or action taken to reimpose UN sanctions “would be incapable of legal effect.”

The United States quit the nuclear deal in 2018.

“We have worked tirelessly to preserve the nuclear agreement and remain committed to do so,” said the UN envoys for Britain, France and Germany, adding that they remain committed to “fully implementing” a 2015 Security Council resolution that enshrines the pact, which also included Russia and China.

US Secretary of State Mike Pompeo said last month that he triggered a 30-day process at the Security Council leading to a return of UN sanctions on Iran on Saturday (0000 GMT Sunday) that would also prevent a conventional arms embargo on Tehran from expiring on Oct. 18.

But 13 of the Security Council members say Washington’s move is void because it is no longer a party to the nuclear deal. The United States say it can make the move because the 2015 Security Council resolution still names it as a participant.

Diplomats say few countries are likely to reimpose the measures, which were lifted under the deal.

The US was left isolated at the UN Security Council last month when it said it wished to reimpose snapback sanctions, with the European nations – known collectively as the E3 – arguing that the US was no longer a participant in the deal and so unable to act unilaterally. The US, which left the deal in 2018, described the E3 position as crackers. A further showdown on the issue at the UN is expected this month.

British Foreign Secretary Dominic Raab hosted a meeting in Kent County with his German and French counterparts Heiko Maas and Jean-Yves Le Drian.

Germany and France, cooperating with the UK over Iran despite the Brexit backdrop, acknowledge there are special UK sensitivities about defying its closest partner the US on such a critical security issue. The European diplomats recognize that the UK wants a free trade deal with the US, and defying the US on snapback may infuriate Donald Trump.

The US also wants the E3 to vote at the UN Security Council against the automatic lifting of the ban on the export of conventional weapons to Iran, but the E3 feels frustrated the US has not yet shown a willingness to negotiate a compromise on the ban’s extent.

A maximalist US stance on the ban will only ensure no ban extension is passed since Russia and China will use their veto at the Security Council, the E3 believes. The ban is supposed to lift automatically in October in line with the five-year timetable set out by the nuclear deal in 2015.

Earlier this week Raab told MPs that the legal advice was reasonably clear that the US had excluded itself from imposing snapback sanctions by leaving the deal in 2018.

In a tweet after the Kent meeting Raab said “we are committed to holding Iran to account”. The German Foreign Office placed a different emphasis saying they “reject the US snapback attempt and remain committed to preserving the nuclear agreement, but Iran urgently needs to return to full compliance”.

The Iranian Foreign Ministry has not officially confirmed reports that the country’s Foreign Minister Mohammad Javad Zarif, under sanctions from the US, will tour Europe next week ahead of the further UN showdown.

Iran’s uranium stockpile is currently 10 times over the limit set in the deal, a leaked UN watchdog report shows, but Iran says it is entitled to breach its obligations due to the failure of the EU to defy the US over improving trade with Iran.

Zarif stresses Iran is fully in compliance with the system of inspections of its nuclear sites. Iran resolved differences with the UN watchdog the International Atomic Energy Agency over its inspectors accessing two sites last month.