BRUSSELS: 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 US 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 US 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 US 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 US 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."

In France, Belgium and the Netherlands, leaders weighed in, criticizing Trump's decision to temporary halt to all refugee admissions, as well as immigration from seven Muslim-majority nations.

Trump's decision "only aims to exacerbate tensions, create potential conflicts with in the end the greatest inefficiency regarding results in the fight against terrorism," French Prime Minister Bernard Cazeneuve said in France's lower house of parliament.

He said that the ban "is useless because it ostracizes some countries, it makes it impossible to welcome people who are persecuted in their country and need protection from free nations."

In the Netherlands, Foreign Minister Bert Koenders hit back, saying, "If you want to fight terror, then the worst thing you can do is trample human rights."

Like Belgium, the Dutch government has updated its travel advisory for the US to warn of the effects of Trump's new policy on citizens who have dual nationality with one of the seven nations affected.

And even royalty got involved. During the very ornate speech to the authorities of the country, King Philippe had unkind words for both the US and Britain.

The monarch said that by looking inward, it "countered their own tradition of openness and generosity."

He said that "you cannot restore confidence by turning back the clock, based on a kind of utopian nostalgia. And you can't do it by building walls."