EU chief Donald Tusk warned on Tuesday 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.
The second threat, said Tusk, was the internal challenge posed by a rise in anti-EU nationalism, and the third was the "state of mind of the pro-European elites" who were too keen to pander to populism to win votes.
EU leaders have become increasingly concerned by Trump's recent ban on migration from seven Muslim countries, plus comments in which he appeared to back Britain's exit from the EU and the eventual break-up of the union.
'Dependence on USA, Russia, China'
All 28 EU leaders will discuss Europe's migration crisis at Friday's summit in Valletta, Malta.
They will then meet without British Prime Minister Theresa May to discuss the future after Brexit, and preparations for a summit in Rome in March to mark the 60th anniversary of the EU's founding treaty.
Tusk added that "only together can EU countries be fully independent. Disintegration will only result in dependence on USA, Russia and China."
"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," he said.
These included securing Europe's borders, boosting the economy and increased cooperation on defence and security, Tusk added.
Europe should in the meantime seek to boost its own trade with the rest of the world even as the United States tightens its frontiers.
"We should use the change in the trade strategy of the US to the EU's advantage by intensifying our talks with interested partners, while defending our interests at the same time," Tusk added.
But he warned that European-American ties were still essential for the future.
"We cannot surrender to those who want to weaken or invalidate the transatlantic bond, without which global order and peace cannot survive," he said.
"We should remind our American friends of their own motto: United we stand, divided we fall."