France, Britain and Germany formally triggered the dispute mechanism in Iran's nuclear deal on Tuesday, the strongest step the Europeans have taken so far to enforce an agreement that requires Iran to curb its nuclear program.The European powers said they were acting to avoid a crisis over nuclear proliferation adding to an escalating confrontation in the Middle East. In a statement, they said they still want the nuclear deal to succeed and were not joining a "maximum pressure" campaign against Iran by the United States, which abandoned the deal in 2018 and has reimposed crippling economic sanctions.Triggering the dispute mechanism amounts to formally accusing Iran of violating the terms of the agreement and could lead eventually to the reimposition of U.N. sanctions that were lifted under the deal.Iran has gradually rolled back its commitments under the accord since the United States quit. Tehran argues that it has the right to do so because of Washington's actions."We do not accept the argument that Iran is entitled to reduce compliance with the JCPoA," the three European countries said in a joint statement, using the formal name of the agreement, the Joint Comprehensive Plan of Action.In triggering the dispute mechanism, "our three countries are not joining a campaign to implement maximum pressure against Iran. Our hope is to bring Iran back into full compliance with its commitments under the JCPoA.”Germany Foreign Minister Heiko Maas told reporters in Berlin it was important “to save this nuclear deal for the future because it prevents Iran from getting a nuclear bomb.”Action had to taken, however, he said, because “we cannot sit back and watch Iran permanently announcing it will no longer stick to its commitments. That's why we chose the instrument of triggering the dispute mechanism, to ensure that Iran declares it will stick to its nuclear deal commitments in the future.”Mass called on Iran to participate in the dispute mechanism. Tehran has long accused the Europeans of reneging on promises to protect its economy from U.S. sanctions. Iran’s Foreign ministry spokesman Abbas Mousavi dismissed the "completely passive action" of the three countries and said Iran would support any act of "goodwill and constructive effort" to save the agreement.To trigger the dispute mechanism, the three European countries notified the European Union, which acts as guarantor of the agreement. EU foreign policy chief Joseph Borrell said the aim was not to reimpose sanctions but to ensure compliance.“There is no alternative to this deal [JCPOA],” Borrell told reporters in Brussels. “We have to preserve the nuclear deal and work to go back to full and effective implementation,” Borrell added.After months of announcing gradual steps to reduce compliance, Iran said on Jan. 6 it would scrap all limits on enriching uranium.The nuclear diplomacy is at the heart of a broader confrontation between Iran and the United States, which killed Iran's most powerful military commander in a drone strike on Jan. 3. Iran has since seen an outpouring of domestic unrest after accidentally shooting down a Ukrainian airliner.The European countries said they were acting "in good faith with the overarching objective of preserving the JCPoA.”"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.U.S. President Donald Trump abandoned the nuclear deal reached under his predecessor Barack Obama, arguing it was too weak and new sanctions would force Iran to accept more stringent terms. Iran says it will not negotiate with sanctions in place.British Prime Minister Boris Johnson said on Tuesday the way forward was to agree a new "Trump deal.”"If we're going to get rid of it, let's replace it and let's replace it with the Trump deal," Johnson said. "President Trump is a great dealmaker, by his own account. Let's work together to replace the JCPoA and get the Trump deal instead."Under the 2015 deal's dispute mechanism, the EU should now inform the other parties - Russia and China as well as Iran - of the European move. There would then be 15 days to resolve differences, a deadline which can be extended by consensus.The process can ultimately lead to a "snapback" - the reimposition of sanctions under previous U.N. resolutions."At one point we have to show our credibility," said a European diplomat. A second diplomat said: "Our intention is not to restore sanctions, but to resolve our differences through the very mechanism that was created in the deal.”Tovah Lazaroff contributed to this report.