The deal, known as the Joint Comprehensive Plan of Action (JCPOA), curbed Iran's nuclear programme in exchange for sanctions relief but has been progressively weakened since the US withdrew from it in 2018.

That withdrawal left the remaining signatories -- Britain, France, Germany, as well as China and Russia -- scrambling to save the deal and mitigate the impact of re-imposed American sanctions.

The three European signatories now say they have "no choice" but to trigger the mechanism -- which experts say could spell the end of the accord.

- What is the dispute resolution mechanism? -

The mechanism allows any party to the deal to refer an issue to a so-called "joint commission" of representatives from all parties.

The joint commission -- due to meet this month in Vienna, according to a diplomatic source -- then has 15 days to resolve the problem, or more if the parties agree.

If the problem is still unresolved, any party can refer it to the signatories' foreign ministers, as well as a three-member advisory board, who again has at least 15 days to solve it.

If not resolved, the joint commission considers the advisory board's opinion for no more than five days.

The last step is notifying the UN Security Council.

UN sanctions would then automatically "snap back" after 30 days unless the Security Council voted to stop this.

The Europeans had evoked the possibility of triggering the mechanism late last year.

"It was a matter of when they were going to do it... Definitely the pressure from the US is growing," Ellie Geranmayeh from the European Council on Foreign Relations said.

- How significant is triggering the mechanism? -

Geranmayeh told AFP that the triggering of the process "set in motion a new phase" with "considerable risk".

She said the current impasse could only be solved if the Europeans presented Iran with an economic package to offset US sanctions -- which so far they have been unable to do to Tehran's satisfaction.

On the other hand, Iran could respond to this "pressure tactic" by in turn further stepping up its nuclear programme and leaving the Non-Proliferation Treaty (NPT), which would be a "major turning point", Geranmayeh said.

Analysts say the dispute resolution mechanism was put in place to deal with technical issues rather than the current wider political impasse.

But diplomats say the mechanism allows for time frames to be extended with a diplomatic source saying it left "a lot of latitude", allowing for "constructive dialogue".

Mark Hibbs, a research fellow at the Carnegie Endowment for International Peace, told AFP that it could be "an opportunity for quiet diplomacy", on one hand, but could also mean the end of the nuclear deal.

- What are the UN sanctions? -

Analysts say the UN sanctions which would be re-imposed would have little impact on the Iranian economy, already under severe strain from measures the US has taken since May 2018.

However, the issue is symbolically important because the UN sanctions included an arms embargo which is due to expire in October 2020.

Iranian President Hassan Rouhani said in November that the lifting of the embargo was a "political and security goal" for Iran.

Conversely, the US and its allies in the region have been keen to maintain the embargo, with US Secretary of State Mike Pompeo branding the October 2020 deadline a "countdown to terror".

Ali Vaez of the International Crisis Group points out that China and Russia have already suggested they will not recognise the re-imposition of UN sanctions, potentially leading to "deepening fissures over this issue".

"UN sanction snapback could exhaust whatever leverage Europe retains without producing any gains; and closing the door on Europe's ability to even mediate tensions between Iran and the US," he wrote.

- Why is time of the essence? -

The 2015 accord aimed to fix Iran's so-called "breakout time" -- the period needed to produce weapons-grade material for a bomb -- to at least a year.

Iran insists it does not want a nuclear weapon.

French Foreign Minister Jean-Yves Le Drian warned Friday that if the Iranians "continue to unravel the Vienna accord, then yes in quite a short period, between one and two years, they could have a nuclear weapon, which is unimaginable".

Iran announced on January 5 that its nuclear programme "no longer faces any limitation in the operational field" extending to its capacity for enriching uranium, the level of enrichment carried out, the amount enriched, and other research and development.

However, Tehran underlined again that any steps would be reversible in the unlikely event of US sanctions being lifted.