Three European powers have written to the UN Security Council detailing Iran's violations of its 2015 nuclear deal, a step diplomats said aimed to pressure Tehran to resolve the issue diplomatically and to avoid reimposing UN sanctions.

The British, French and German letter did not explicitly threaten to "snap back" United Nations sanctions, but it noted that UN Security Council Resolution 2231, which enshrined the nuclear deal and provided that power, expires on Oct. 18, 2025.

In its own letter, Iran rejected the European stance, noting then-US President Donald Trump reneged on the nuclear deal in 2018 and re-imposed US economic sanctions on Iran, arguing they were within their rights to expand their nuclear work.

The effort by Britain, France and Germany, known informally as the E3, to ramp up pressure was also highlighted this week at the International Atomic Energy Agency, where they successfully pushed a resolution critical of Iran despite US reservations.

The E3 letter, which was dated June 3, referred to a report by the UN nuclear watchdog, IAEA, last month that cited Iran's nuclear advances violating the 2015 deal, including by expanding its stockpile and production rates of high enriched uranium. Iran, which has stockpiled more than 100 kilograms of 60% enriched uranium, has enough fissile materials for 3-4 nuclear warheads if it decides to produce weapons.

However, Tehran’s overall reaction to the censure at the Board of Governors meeting of the International Atomic Energy Agency, has been relatively low-key so far.

That 2015 nuclear deal, struck with the E3, China, Russia and the United States, limited Iran's ability to enrich uranium, a process that can yield fissile material for nuclear weapons. In return, the US, UN and European Union eased sanctions on Iran.

Tensions with Iran have increased since the Iranian-backed Hamas militant group attacked southern Israel on Oct. 7, other Iranian proxies have attacked US, Israeli and other Western targets, and Tehran has accelerated its nuclear program while limiting the UN nuclear watchdog's ability to monitor it.

"Iran's nuclear escalation has hollowed out the JCPOA, reducing its nonproliferation value," said the E3 letter seen by Reuters, referring to the 2015 deal formally called the Joint Comprehensive Plan of Action.

"Iran's decision to take remedial measures was in full accordance with its inherent right ... in reaction to the United States' unlawful unilateral withdrawal," Iran's UN ambassador said a June 5 letter seen by Reuters.

Reuters quoted Western diplomats and other sources familiar with the E3 letter as saying that its purpose was to try to raise pressure on Iran within the Security Council and to buy time for a diplomatic solution before next year's expiry of their "snap back" power to reimpose UN sanctions on Iran.

The aim is to "take stock of Iran's nuclear advances, which have become unacceptable and are getting worse, and to increase pressure within the Security Council," said a source familiar with the letter.

UN Secretary-General Antonio Guterres reports twice a year to the Council - traditionally in June and December - on the implementation of the 2015 resolution. The Security Council is due to discuss his next report on June 24.

Daryl Kimball, executive director of the nonprofit Arms Control Association, told Reuters the letter may be a way to lay the groundwork for an eventual "snap back" of UN sanctions, though he stressed that a diplomatic solution is still possible.

"The E3 reference to the October 2025 date, when the option to snap back UN sanctions expires, and their communication to the Security Council suggest they are simply trying to establish the legal basis for possibly snapping back sanctions on Iran at a later point," said Kimball.

"However, such an option, especially in the wake of the US withdrawal from the JCPOA, would likely not move Iran to cooperate but lead it to escalate, perhaps even by withdrawing from the nuclear Non-proliferation Treaty," he said.

With reporting by Reuters