He said on Twitter: "Looking at latest decisions of @realDonaldTrump someone could even think: with friends like that who needs enemies".
Mr Tusk said that confronted with challenges such as Mr Trump, Russia and China, the European Unionneeded to be united "economically, politically and also militarily like never before" or risk being "a pawn" in global politics. "He has made us realise that if you need a helping hand, you will find one at the end of your arm", Tusk said.
European officials sounded off on the Trump administration's Iran policy on Wednesday, amid signs European companies are prepared to end business dealings with the country instead of incurring possible USA sanctions.
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.
Tusk even compared the USA administration to Europe's traditional foes Moscow and Beijing as he launched his broadside hours before a dinner of the 28 leaders in Sofia where they will discuss the issue.
Tusk went on to say the Iran deal remains "good for European and global security, which is why we must maintain it - despite the USA hesitation".
"I have no doubt that in the new global game", Tusk said, "Europe will either be one of the major players or will be a pawn". "This is the only real alternative".
The EU and the United States have traditionally been the closest of allies, working together also via North Atlantic Treaty Organisation.
European Commissioner Dimitris Avramopoulos said the EU was willing to a so-called blocking statute that would make it illegal for European companies to comply with U.S. sanctions imposed on Iran and not recognize USA court decisions that enforce American sanctions.
Merkel however said that Europe had no choice but to stick with the ties that have bound it to Washington since World War II.
The EU's 28 leaders are gathering in the Bulgarian capital for a dinner discussion on Wednesday on how to salvage the Tehran accord and European business dealings with Iran, from Trump's sanctions to how to avoid a trade war in an escalating tariff dispute with the United States.
The remarks come as in their first meeting since the United States withdrawal from the JCPOA, EU leaders were expected to meet to explore options on Wednesday for keeping the Iran nuclear deal alive and protecting their reviving economic cooperation with Tehran.
Under Trump, the US has walked out of the global Paris climate deal, is threatening to impose punitive tariffs based on national security on an ally like the European Union, and has stirred Middle East politics by pulling out of the Iran nuclear deal and moving the USA embassy in Israel to Jerusalem.
The opening of the United States Embassy in Jerusalem coincided with Palestinian demonstrations in which Israeli soldiers killed 60 protesters and wounded thousands of others - the bloodiest day in the Gaza Strip since 2014.