But the reach of the US financial system, the dominance of the dollar and the presence of European companies' operations in the United States all weaken any potential EU measures. He said: "We are witnessing today a new phenomenon: the capricious assertiveness of the American administration".
"Looking at latest decisions of [Trump] someone could even think: with friends like that who needs enemies", wrote Donald Tusk, president of the European Council.
Washington and Brussels have been at loggerheads over a number of issues since Mr Trump was inaugurated in January 2017, most recently a trade war of aluminium and steel tariffs and the Iran nuclear deal.
The discussions, which officials said were long but with "no real controversy", stuck to the issues of the day and avoided a wider questioning of the EU's relations under Trump, despite Tusk's tough words.
The 28 anxious EU leaders are gathering in the Bulgarian capital for discussions on how to salvage the nuclear deal and European business dealings with Iran from Trump's sanctions.
He also warned that European Union nations should prepare to confront USA sanctions that could hit their firms doing business with Iran.
The European Commission is refusing to open talks on wider trade terms with the U.S., including import tariffs on cars - a bugbear of the United States president - until it receives a permanent exemption from punitive taxes on European steel and aluminium.More news: Lava, ash spew from Hawaii's Kilauea volcano
Tusk said the European Union must be more united than ever before to face off against Trump's "capricious assertiveness". To put it simply: either we are together, or we will not be at all.
But the 28 EU leaders did not make any quick decisions during their first meeting on the matter since Trump quit the accord earlier this month, highlighting how USA clout in global trade and finance limits the Europeans' scope for action. He has made us realise that if you need a helping hand, you will find one at the end of your arm.
"Europe must do everything in its power to protect, despite today's mood, the transatlantic bond".
The 28 European Union leaders meeting in Sofia were not expected to reach any quick conclusions on how to try to shield their economic cooperation with Iran.
The European Council president repeated his comments in a Tweet tagging the United States president.
Theresa May will travel to the Bulgarian capital where she will join a working dinner with her fellow European leaders, including European Commission President Jean-Claude Juncker.
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. The EU and U.S. are friends and partners, therefore USA tariffs can not be justified on the basis of national security'.
"The EU and USA are friends and partners". Trump's decision to pull out of the Paris climate change accord and the 2015 nuclear deal with Iran threaten Europe's own foreign policy, which was long complementary to Washington's.