Europe’s threat to trigger a mechanism that could reimpose United Nations sanctions on Iran marks a significant breakdown in diplomacy to try to save the 2015 nuclear deal and could presage its death knell, diplomats say.

Britain, France and Germany have sought to salvage the pact, under which Iran undertook to curtail its uranium enrichment program in return for relief from sanctions crippling its economy, since the United States withdrew last year.

But the three European powers have failed to make good on the trade and investment dividends promised to Iran under the deal as they have been unable to shield Tehran from renewed U.S. sanctions that have strangled its vital oil trade.

That has prompted Iran to renege step by step from its nonproliferation commitments under the deal.

The U.N. nuclear watchdog (IAEA) confirmed Monday that Iran had resumed enriching uranium in its underground Fordow plant and was rapidly accelerating enrichment with a variety of advanced centrifuge machines also banned by the deal.

The move has alarmed European powers that had previously dismissed Tehran’s breaches, such as exceeding the cap on stockpiles of enriched uranium and on the fissile purity of enrichment, as insignificant and reversible.

Britain, France, and Germany raised the prospect of a restoration of international sanctions for the first time late Monday after a meeting of foreign ministers in Paris, saying they were ready “to consider all mechanisms … including the dispute resolution mechanism.”

Under the terms of the 2015 deal, if any party believes another is not upholding their commitments they can refer the issue to a Joint Commission comprising Iran, Russia, China, the three European powers, and the European Union.

If the complaining party cannot resolve the matter at the Commission level, it could then notify the U.N. Security Council, which must vote within 30 days on a resolution on continuing Iran’s sanctions relief.

If this is not adopted within that time span, sanctions that were in place under previous U.N. resolutions would be reimposed — known as a “snapback” — unless the Council decided otherwise.

“We don’t want to pull out of the (deal) too soon, but equally we cannot sit back. The Russians and the Chinese are not going to trigger this, but us, as Europeans, will have to take a stance at some point,” said a European diplomat.

“It is not if but when, unless Iran pulls back, but even then, they are gaining (nuclear) knowledge by spinning these centrifuges, so we have to react.”

Enrichment is the main pathway towards producing fuel that detonates a nuclear bomb, though Iran has refined to a fissile purity of 4.5 percent — roughly suitable for electricity generation — far below the bomb-grade threshold of 90 percent.

European disquiet has been exacerbated by the detention of an IAEA inspector at Iran’s Natanz nuclear facility at the end of October, contrary to stipulations in the deal that Iran must grant daily access to the site as requested.

Diplomats said a meeting of the parties to the accord would take place over the next week to discuss latest developments. However, they say, the Europeans are unlikely to trigger the mechanism before January when Iran is due to announce its next round of steps away from compliance with the deal.

“What we’re now seeing is the dismantling of the JCPOA,” said another diplomat, referring to the Joint Comprehensive Plan of Action, as the agreement is formally known.

“We haven’t decided on launching the mechanism, because we need to be sure of how it will help us in trying to defuse tensions. The questions we’re asking is when, how, and whether it benefits us to do it?”

Iranian Foreign Minister Mohammad Javad Zarif reiterated Tuesday that Iran had itself launched the complaint mechanism and it was ceasing to heed its deal commitments because the Europeans had failed to protect it from U.S. sanctions.

The deal’s objective was to extend the time Iran would need to accumulate enough fissile material for an atom bomb, if it sought one — something sometimes referred to as “breakout time” to about a year from 2-3 months.

“We’re now entering a phase where Iran’s actions have a serious impact on the breakout time,” said another EU diplomat, adding that the window to bring Iran and the United States back to the negotiating table was very small now.