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 worldwide community.
Donald Trump has been pressing Europe to leave the nuclear deal ever since he unilaterally took the United States out of deal in May 2018, and the White House will be delighted that its sustained pressure has paid off. This month it said it would abandon limits on its production of enriched uranium, a step the Europeans said was likely to force them to respond.
The three European powers party to the deal act though the EU, which coordinates any negotiations over it with the other parties, which include Russian Federation and China.
France, Germany and the United Kingdom said they did not accept Iran's argument.
Activating the mechanism - which is only possible if one or more signatories suspect a non-compliance with the deal - could eventually lead to the UN Security Council deciding whether to bring back sanctions against Iran.
"At one point we have to show our credibility", said one diplomat referring to Iran's recent nuclear decisions.
"Our intention is not to restore sanctions, but to resolve our differences through the very mechanism that was created in the deal", a second diplomat said.
Last Sunday, French President Emmanuel Macron, German Chancellor Angela Merkel and UK Prime Minister Boris Johnson said in a joint statement that they remained committed to the Joint Comprehensive Plan of Action (JCPOA) and urged Tehran to cancel all measures that go against the Iran nuclear deal. The move came after the U.S. killed top Iranian general Qassem Soleimani in an airstrike in Baghdad. He added that there is no alternative to the Iran nuclear deal.
After the killing of Soleimani, Iran targeted US forces in Iraq, apparently without any casualties.
Britain's Johnson, a conservative who has pitched himself as an ally of Trump who can help bridge the diplomatic gap between the United States and Europe, said he wanted to avoid a military conflict with Iran. "Let's work together to replace the JCPOA and get the Trump deal instead", Johnson told the BBC.
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Air pollution was at hazardous levels across the city due to thick bushfire smoke, and people are being told not to go outside. Earlier on Tuesday, the EPA's website crashed after receiving a "significant amount of traffic".