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

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

Though Europe pledged continued support, the mechanism to offset US sanctions never effectively materialised, forcing many Western firms to exit Iran and deepening its economic crisis.

"Iran holds the European parties responsible for negligence in implementing the agreement," said foreign ministry spokesman Esmaeil Baqaei ahead of Friday's talks in Istanbul with Britain, France and Germany on the deal's future.

Tehran will also host a trilateral meeting Tuesday with Chinese and Russian representatives to discuss the nuclear issue and potential sanctions.

The Chinese foreign ministry said Beijing would "continue to play a constructive role in pushing relevant sides to restart dialogue and negotiations, and reach a solution that takes in account the legitimate concerns of all parties".

In recent weeks, the three European powers have threatened to reimpose international sanctions on Tehran, accusing it of breaching its nuclear commitments.

Germany said the Istanbul talks would be at the expert level, with the European trio working "flat out" to find a sustainable and verifiable diplomatic solution.

"If no solution is reached by the end of August... the snapback also remains an option for the E3," said its foreign ministry spokesman, Martin Giese.

A clause in the 2015 agreement allows for UN sanctions on Iran to be reimposed through a "snapback" mechanism in the event of non-compliance.

However, the agreement expires in October, leaving a tight deadline.

- 'No intention of speaking with America' -

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 enrichment required for a nuclear weapon.

Using the snapback clause was "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," he said.

Western powers -- led by the United States and backed by Israel, Iran's arch-enemy -- have long accused Tehran of secretly seeking nuclear weapons capability.

Iran has repeatedly denied this, insisting its nuclear programme is solely for civilian purposes such as energy production.

Tehran and Washington had held five rounds of nuclear talks since April, but a planned meeting on June 15 was cancelled after Israel launched strikes on Iran on June 13, triggering a 12-day conflict.

"At this stage, we have no intention of speaking with America," Baqaei said Monday.

Israel launched on June 13 a wave of surprise strikes on its regional nemesis, targeting key military and nuclear facilities.

The United States launched its own set of strikes against Iran's nuclear programme on June 22, hitting the uranium enrichment facility at Fordo, in Qom province south of Tehran, as well as nuclear sites in Isfahan and Natanz.