EU warns Iran nuclear deal at 'critical juncture'
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"That risks undermining diplomatic efforts, including ours, to facilitate a US return to the JCPOA and to bring back Iran to the full implementation of its JCPOA commitments," said the letter sent ahead of a meeting of the top diplomats on Monday.
The JCPOA, or Joint Comprehensive Plan of Action, is the instrument agreed to implement the 2015 accord.
The EU and other signatories to the pact are pushing to get US President-elect Joe Biden to return to the agreement after scrambling to keep it alive following Donald Trump's withdrawal over two years ago.
The Iranian government has signalled a readiness to engage with Biden, who takes office on January 20 and has himself expressed willingness to return to diplomacy with Tehran.
But supreme leader Ayatollah Ali Khamenei has insisted the US must start out by lifting its sanctions on Tehran.
That move came after Iran announced earlier in January that it had started the process of enriching uranium to 20 percent.