The 2015 deal, reached between Iran and the UN Security Council's permanent members -- Britain, China, France, Russia and the United States -- plus Germany imposed curbs on Iran's nuclear programme in exchange for sanctions relief.

But it unravelled when the United States in 2018, during Donald Trump's first term as president, unilaterally withdrew from the accord and reimposed sweeping sanctions.

The Europeans had pledged continued support for the deal, but the mechanism intended to offset US sanctions never materialised effectively and many Western firms were forced to exit Iran, which has since faced a deepening economic crisis.

"The European parties have been at fault and negligent in implementing" the nuclear agreement, said Iranian foreign ministry spokesman Esmail Baqaei.

The International Atomic Energy Agency says Iran is the only non-nuclear-armed country currently enriching uranium to 60 percent -- far beyond the 3.67 percent cap set by the 2015 accord.

That is a short step from the 90 percent required for a nuclear weapon.

In recent weeks, France, Britain and Germany have threatened to trigger the UN snapback mechanism to reimpose sanctions on Tehran, accusing Iran of breaching its nuclear commitments.

Using this clause "is meaningless, unjustifiable and immoral", Baqaei told a news conference, arguing that Iran only began distancing itself from the agreement in response to Western non-compliance.

"Iran's reduction of its commitments was carried out in accordance with the provisions outlined in the agreement," the foreign ministry spokesman added.