Council president Donald Tusk urged leaders meeting in Bulgaria to form a "united European front" against Trump's withdrawal from the Iranian nuclear deal and his move to impose trade tariffs on Europe. But the 28 EU leaders are not expected to make any quick decisions during their first meeting on the matter since Trump quit the accord, highlighting how USA clout in worldwide trade and finance limits the Europeans' scope for action.
Trade tensions have been growing since March 23, when Donald Trump ordered to introduce a 25% tariff on steel imports and 10% tax on aluminum, claiming to protect the USA producers from unfair competition and boost national security.
With member states yet to agree on how to handle the challenges, Mr Tusk called on leaders gathering in the Bulgarian capital, Sofia, to unite behind the tough line being taken by key European actors in response to the White House's actions.
Mr Tusk said: "Besides traditional political challenges, such as the rise of China or the aggressive stance of Russian Federation, we are witnessing today a new phenomenon: the capricious assertiveness of the American administration".
Under the deal, Iran agreed to freeze its nuclear programs in return for the lifting of most global sanctions.
The head of the bloc's executive, Jean-Claude Juncker of the European Commission, has also presented options the leaders have to shield European investments in Iran and the slowly reviving economic cooperation, which many EU states hope to benefit from.
"Looking at the latest decisions of President Trump some could even think, 'With friends like that, who needs enemies?'"
Previously in March Tusk offered a more conciliatory stance on the economic differences between the U.S. and European Union, urging Trump to "go back" to trade talks instead of waging a "trade war".
The EU will continue to support the Iran deal "as long as Iran respects the deal" but will address USA concerns over Iran's role in the Middle East crisis and its ballistic missile programme.
"The way to address will give answer to the broader question on how Europe should deal with the Trump administration", a senior European Union official had noted before the meeting.
British Prime Minister Theresa May, German Chancellor Angela Merkel and French President Emmanuel Macron - whose countries are the signatories to the Iran deal along with the US, Russia and China, gave their views on the situation to their colleagues, he said.
"Despite all the difficulties we have these days, the transatlantic relations are and will remain of outstanding importance", Merkel told the German parliament.
"I will propose we stick to our guns", Tusk said.
"The extra-territorial dimension of U.S. sanctions is unacceptable".
"The EU and USA are friends and partners".
The EU has called for "utmost restraint" after Israeli forces killed 60 Palestinians during clashes and protests along the Gaza border against the opening of the USA embassy in Jerusalem, in the conflict's bloodiest day in years.
The EU's top energy and climate official, Commissioner Miguel Arias Canete, is heading to Iran on May 18-21 for talks on energy cooperation, a symbolic gesture from the EU that it wants to stay engaged despite the USA withdrawal.