The European signatories to a nuclear deal signed between Iran and world powers in 2015 have triggered a diplomatic "dispute mechanism", in their strongest response yet to Tehran's steps away from the unravelling pact.

Following Washington's decision to withdraw from the deal in May 2018, Iran began dropping its commitments under the deal, formally known as the Joint Comprehensive Plan of Action (JCPOA).

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On January 6, days after the US assassination of a top Iranian general, Tehran took a further step by announcing it would scrap limits on enriching uranium, though it said it would continue cooperating with the United Nations nuclear watchdog.

"We do not accept the argument that Iran is entitled to reduce compliance with the JCPOA," France, Germany and the United Kingdom said in a joint statement on Tuesday, adding they had no choice but to trigger the process that could eventually lead to UN sanctions.

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"Instead of reversing course, Iran has chosen to further reduce compliance," the statement said.

Iran has dismissed the European move, but said it would be willing to consider efforts to bolster the fragile deal.

"The Islamic Republic of Iran, as in the past, has complete readiness to support any [act of] goodwill and constructive effort to save this important international agreement," Foreign Ministry spokesman Abbas Mousavi said in a statement.

Josep Borrell, the European Union's foreign policy chief, said the aim of the dispute mechanism was not to reimpose sanctions, but "to find a solution for the return to full compliance" with the deal, which was also signed by Russia and China.

Russia condemned the EU move, warning it risked causing a "new escalation".

The foreign ministry said in a statement: "We do not rule out that the thoughtless actions of the Europeans could lead to a new escalation around the Iranian nuclear accord."

The ministry added that Moscow sees "no reason" for such a move.

"Despite all the challenges the Iranian nuclear accord has not lost its relevance," the ministry concluded.

Diplomatic difficulties

The accord - deemed at the time to be a landmark achievement - sought to prevent Iran from developing nuclear weapons in exchange for sanctions relief and global powers beginning to welcome the country back into the international community.

But US President Donald Trump called it "the worst deal in history" and in May 2018 unilaterally withdrew from the deal, re-imposing crippling sanctions which have devastated Iran's economy.

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In response, Tehran challenged the European powers to fulfil their part of the deal and boost its economy. It began to raise the purity to which it enriched uranium beyond the limits it agreed to in the 2015 deal to apply further pressure on Europe, as well as stockpiling nuclear material.

"It means that [for European powers] there are serious problems with the implementation of this nuclear agreement on the part of the Iranians - and Iranians feel the same way about the Europeans, that they're not upholding their end of the deal, which is why they all find themselves in this position now," said Al Jazeera's Dorsi Jabbari, reporting from Tehran.

"Iran is in a very particular position right now, given the latest chain of events that have unfolded since the beginning of this year... This now means that there is further pressure on the Rouhani government, whose main achievement was the securing of this nuclear deal in 2015.

"But since the US left and reimposed a series of sanctions, there have been no benefits to the deal for the Iranian people - that's why they feel like they haven't been able to see why they're still in it.

"Everything has changed since the US left and this whole agreement is now in question... I think this mechanism has been triggered to force all sides to come back to the negotiating table and talk out the issues they all have at the moment."

With the accord under intense pressure, despite the statements of European leaders, support appears to be growing for a new pact which could receive the endorsement of the US president.

A new deal?

United Kingdom Prime Minister Boris Johnson said on Tuesday he would be willing to work on a "Trump deal" to replace the international accord agreed between Iran, the five permanent members of the UN Security Council, plus Germany.

"If we are going to get rid of it then we need a replacement," Johnson said. Britain and other European powers have been trying to salvage the deal since President Donald Trump pulled the United States out.

Johnson said a "Trump deal" would be "a great way forward", but did not specify the details of the proposal.

His idea stands at odds with Tuesday's statement from Britain, France and Germany, which expressed "determination to work with all participants to preserve" the deal.

"From the American perspective it's a flawed agreement, it expires, plus it was negotiated by (former) President Obama," Johnson said.

"President Trump is a great deal-maker - by his own account and many others. Let's work together to replace the JCPOA and get the Trump deal instead."

In an apparent bid to keep the door open for diplomacy, the three European signatories - known as the E3 - said they were not joining the US campaign to implement "maximum pressure" against Iran.

"Given recent events, it is all the more important that we do not add a nuclear proliferation crisis to the current escalation threatening the whole region," they said.