The European Union's diplomatic chief on Monday voiced regret at Tehran's latest step away from the beleaguered 2015 Iran nuclear deal, as the Islamic republic seethes over the killing of one of its top commanders in a US air strike.

Josep Borrell, the EU's high representative for foreign affairs, tweeted that the accord, which has been teetering on the brink of collapse since US President Donald Trump unilaterally withdrew the United States, was "now more important than ever".

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European-led efforts to keep Iran in the Joint Comprehensive Plan of Action (JCPOA) - which curbed Tehran's nuclear programme in return for sanctions relief - have borne little fruit since the US withdrew in May 2018 and reimposed punishing sanctions.

In line with the usual EU policy, Borrell said the bloc would wait for further details of any Iranian breaches from the International Atomic Energy Agency (IAEA) before deciding how to respond.

"Deeply regret Iran's latest announcement on #JCPOA. As ever we will rely on @iaeaorg verification," tweeted Borrell, Spain's former foreign minister.

"Full implementation of #NuclearDeal by all is now more important than ever, for regional stability & global security. I will continue working with all participants on way forward."

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Breaking the deal

On Sunday, Iran said it would forego the "limit on the number of centrifuges" it had pledged to honour in the 2015 agreement, casting doubt on an EU push for talks to salvage the deal.

Borrell spoke to Iranian Foreign Minister Mohammad Javad Zarif at the weekend and issued a personal invitation to come to Brussels, but Iran has not given a public response.

EU spokesman Peter Stano said there was "a lot of activity going on" as the EU seeks to help defuse tensions that heightened dramatically on Friday when a US drone strike killed Qassem Soleimani, the veteran commander of the Iran's Islamic Revolutionary Guard Corps' foreign operations, at Baghdad airport.

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European Commission President Ursula von der Leyen is expected to make a statement on the situation later on Monday, while NATO has convened an extraordinary meeting of its ruling North Atlantic Council to discuss the crisis - in particular on the future of its training mission in Iraq.

Germany has suggested EU foreign ministers should meet in Brussels later this week. A decision on this could be made at a regular gathering of EU ambassadors on Tuesday.

Iran's 2015 nuclear accord with the United Nations Security Council's five permanent members - Britain, China, France, Russia and the United States - plus Germany, has been hanging by a thread since Trump withdrew.

Tehran said it would continue cooperating "as before" with IAEA inspectors, but the leaders of Germany, France and Britain reacted by urging Iran to rethink its announcement.