EU chief Donald Tusk warned yesterday that US President Donald Trump's administration was a "threat" facing the bloc along with China, Russia and radical Islam.

In a strongly worded letter to EU leaders ahead of a summit in Malta, Tusk said the bloc must take "spectacular steps" to stay together and should take advantage of Trump's isolationism to boost trade with other countries.

The former Polish prime minister said in the wake of the Brexit vote the European Union faced "three threats" that made the 28-nation bloc's situation "more dangerous than ever before".

"The first threat, an external one, is related to the new geopolitical situation in the world and around Europe," European Council president Tusk said in the letter obtained by AFP.

"An increasingly, let us call it, assertive China, especially on the seas, Russia's aggressive policy towards Ukraine and its neighbours, wars, terror and anarchy in the Middle East and in Africa, with radical Islam playing a major role, as well as worrying declarations by the new American administration all make our future highly unpredictable," Tusk said.

"Particularly the change in Washington puts the European Union in a difficult situation; with the new administration seeming to put into question the last 70 years of American foreign policy," Tusk added.