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BRUSSELS — President Donald Trump has joined Russia, China and radical Islam as a threat to Europe, the president of the European Council Donald Tusk said on Tuesday.

In a letter to national leaders before a summit that he will chair in Malta to prepare the European Union's future after Britain leaves, the conservative former Polish prime minister called on Europeans to stick together to avoid domination by three other continental powers.

European Council President Donald Tusk. STEFANIE LOOS / AFP - Getty Images

Trump's more protectionist trade policy offered the E.U. a chance and it should do more now to set up free trade deals, he added.

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Saying the E.U. faces its biggest challenges in its 60-year history, the senior official said an "assertive China," "Russia's aggressive policy" toward its neighbors, "radical Islam" fueling anarchy in the Middle East and Africa were key external threats. These, he said, "as well as worrying declarations by the new American administration, all make our future highly unpredictable."

Tusk's remarks were among the strongest directed at the new U.S. president since Trump took office 11 days ago and reflects a growing sense in many European capitals of a need to respond to his policy moves, notably the ban at the weekend on the entry of refugees and others from seven Muslim-majority countries.

Leaders in Brussels have been particularly concerned that Trump has supported Brexit and spoken of other countries following Britain out of the bloc.

"The disintegration of the European Union will not lead to the restoration of some mythical, full sovereignty of its member states, but to their real and factual dependence on the great superpowers: the United States, Russia and China," Tusk wrote to the E.U. leaders. "Only together can we be fully independent.

"We must therefore take assertive and spectacular steps that would change the collective emotions and revive the aspiration to raise European integration to the next level."

European diplomats said senior national officials and diplomats discussed a possible E.U. response to Trump at a meeting in Brussels on Monday. However, some governments were cautious that Europeans should not be hasty to alienate a key ally.