"Looking at the latest decisions of President Trump, someone could even think with friends like that who needs enemies," Tusk told reporters in the Bulgarian capital Sofia before EU leaders met to discuss a "united front" on Trump.

"But frankly speaking, Europe should be grateful to President Trump, because thanks to him we have got rid of all illusions. He has made us realise that if you need a helping hand you will find one at the end of your arm," the European Council president added.

The 28 European Union leaders are meeting for dinner on the eve of a summit to discuss closer ties with the Western Balkans, but tensions with Trump have hijacked the agenda.

European ministers met with a top Iranian official in Brussels on Tuesday in a bid to save the Iranian nuclear accord after Trump decided to pull out. Meanwhile the EU is still trying to win exemptions from tariffs on steel and aluminium exports.

In a scathing attack, former Polish prime minister Tusk compared the Trump administration with Europe's long-time adversaries, Moscow and Beijing.

"Besides the traditional political challenges such as the rise of China or the aggressive stance of Russia, we are witnessing today a new phenomenon: the capricious assertiveness of the American administration," Tusk said.

He called for more unity in the divided EU -- which is set to lose Britain as a member next year -- to face the growing challenges.

"I have no doubt that in the new global game, Europe will either be one of the major players, or a pawn. This is the only real alternative."