European powers on Thursday rejected "ultimatums" from Tehran but vowed to fight to save the Iran nuclear deal, as tensions with the US soar.
Iran said it would defy some restrictions agreed under the 2015 accord and threatened to go further if Europe, China and Russia fail to deliver sanctions relief within 60 days.
Tehran says it is responding to unilateral US sanctions imposed by Washington after President Donald Trump ripped up what he called a "horrible" deal, dealing a severe blow to the Iranian economy.
Europe has stressed the importance of the deal -- in which Iran agreed to curb its nuclear ambitions in return for sanctions relief -- for its own security, and EU president Jean-Claude Juncker said it would be on the agenda at Thursday's summit in the Romanian town of Sibiu.
EU diplomatic chief Federica Mogherini and France, Germany and Britain -- the three European signatories to the deal -- voiced "great concern" at President Hassan Rouhani's dramatic intervention.
"We strongly urge Iran to continue to implement its commitments under the JCPOA in full as it has done until now and to refrain from any escalatory steps," they said in a joint statement, referring to the Joint Comprehensive Plan of Action.
"We reject any ultimatums and we will assess Iran's compliance on the basis of Iran's performance regarding its nuclear-related commitments under the JCPOA."
"We still think that the deal with the Iranians was a chance to bring Iran out of isolation. But of course we realise and we see that the US has a totally different approach and that is why it will stay a difficult issue."
But their efforts have borne little fruit so far, with Iran's Supreme Leader Ayatollah Ali Khamenei dismissing them as a "bitter joke."