Britain, France and Germany accused Iran of developing nuclear-capable ballistic missiles in a letter to UN Secretary-General Antonio Guterres that Iranian Foreign Minister Javad Zarif angrily dismissed on Thursday (December 5th).
UN ambassadors for the three countries said Iran's actions were "inconsistent" with the UN resolution enshrining a 2015 nuclear deal with Iran.
The letter referred to footage shared on social media in April of the test flight of a new Shabab-3 medium-range ballistic missile variant that was "technically capable of delivering a nuclear weapon".
The European powers also pointed to three other launches this year, including that of the Borkan-3, a new medium-range ballistic missile tested by the Iran-backed Houthis (Ansarallah) in Yemen on August 2nd.
The letter, dated November 21st, said these flights were "the latest in a long series of advances in Iranian ballistic missile technology".
Iran's UN ambassador responded in a letter saying the European powers were using "unreliable sources" and "outdated reports" to make misleading arguments.
"Iran is determined to resolutely continue its activities related to ballistic missiles and space launch vehicles, both of which are within its inherent rights under international law," the letter said.
Crunch talks in Vienna
The remaining signatories to the 2015 Iran nuclear deal said they remain committed to the faltering accord, following crunch talks in Vienna Friday as Tehran has vowed to continue to breach limits on its nuclear programme.
Envoys from Britain, France, Germany, China, Russia and Iran took part in the three-hour-long meeting, which is the first time the six parties have gathered in this format since July.
Iran has taken a series of measures, including stepping up uranium enrichment, in breach of the 2015 deal, with another such move likely in early January.
Since last month, European members have begun raising the possibility of triggering the so-called "dispute resolution mechanism" foreseen in the accord, which could lead to the resumption of UN sanctions on Iran.
Despite the mounting tension, Chinese delegation head Cong Fu told reporters after the talks that all parties remained committed to the deal and the dispute resolution mechanism was not invoked.
Analysts say if UN sanctions are re-imposed and the deal falls apart, Iran could also withdraw from the Treaty on the Non-Proliferation of Nuclear Weapons.
Ali Vaez of the International Crisis Group warned the risk of the deal collapsing was increasing as Iran was "running out of measures that are easy to reverse and non-controversial".
The dispute resolution mechanism in the deal has numerous stages, but it can eventually culminate in the UN Security Council voting on whether Iran should still have relief from sanctions lifted under the deal.