The leader of the European Union put longtime ally the United States in a "threat" category on Tuesday, insisting that President Donald Trump is contributing to the "highly unpredictable" outlook for the bloc.

In a letter to 27 EU leaders before Friday's summit in Malta, EU President Donald Tusk mentioned the Trump administration as part of an external "threat" together with China, Russia, radical Islam, war and terror.

Echoing statements from many European capitals, he said that those global challenges, "as well as worrying declarations by the new American administration, all make our future highly unpredictable."

He said that "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."

This year marks the centennial of the U.S. entry in World War I, and it marked the beginning of the American century as well as the enduring trans-Atlantic bond with Europe. Tensions have risen though since Trump was elected U.S. president.

Trump has questioned the NATO alliance linking North America and Europe, and hopes for a major trans-Atlantic trade deal have already taken a deep dive amid worries of U.S. protectionism.

"We should remind our American friends of their own motto: United we stand, divided we fall," Tusk said in the letter and also told a news conference in Tallinn, Estonia, after meeting with the three Baltic prime ministers before the Malta summit.

Britain wasn't part of the letter since it is poised to leave the EU and is only scheduled to attend part of the leaders' meeting in La Valletta. The decision to leave was the biggest setback for the EU in decades, and Trump didn't endear himself with many EU leaders by saying that Brexit "will be a tremendous asset and not a tremendous liability."

Tusk wrote to the leaders that "in politics, the argument of dignity must not be overused," before adding that "today we must stand up very clearly for our dignity, the dignity of a united Europe — regardless of whether we are talking to Russia, China, the U.S. or Turkey."

And Tusk further insisted that any disintegration wouldn't be beneficial to the restored nation states, but instead lead to "their real and factual dependence on the great superpowers: the United States, Russia and China."