The countries' foreign ministers spoke late Monday with the EU's top diplomat Kaja Kallas and "called on Iran to return, as soon as possible and without preconditions, to the negotiating table" over its nuclear programme, the source said on condition of anonymity.

France's Jean-Noel Barrot, Britain's David Lammy and Germany's Johann Wadephul also "called on Iran to avoid any headlong rush against Western interests, any regional expansion and any nuclear escalation" such as non-cooperation with the International Atomic Energy Agency (IAEA), the source said.

The comments came after US President Donald Trump abruptly left a G7 summit in Canada a day early, without spelling out a reason.

Before his departure, the leaders of the club of industrial democracies -- Britain, Canada, France, Germany, Italy, Japan and the United States -- had called for "de-escalation" and stated their conviction that Iran "can never have a nuclear weapon".

Earlier in the day, Trump had taken to social media to back Israel and issue a warning to Tehran, a city of nearly 10 million people.

"Everyone should immediately evacuate Tehran", he wrote, without offering further details.

He later posted that his early exit from the summit had "nothing to do" with working on an Israel-Iran ceasefire but was "much bigger than that".