BRUSSELS, Jan. 31 (Xinhua) -- European Council President Donald Tusk on Tuesday branded U.S. President Donald Trump's "worrying declarations" as an external threat, which, along with other factors, "make our future highly unpredictable."

"Particularly the change in Washington puts the European Union (EU) in a difficult situation, with the new administration seeming to put into question the last 70 years of American foreign policy," Tusk said in a letter to leaders of EU member states ahead of the Valletta Summit slated for Friday.

Vowing that the EU would not "surrender to those who want to weaken or invalidate the transatlantic bond," he said "we should remind our American friends of their own motto: United we stand, divided we fall."

The EU is at odds with the Trump administration over an executive order banning entry to the United States of citizens from seven Muslim-majority countries.

Under the executive order signed by Trump on Friday, refugees from all over the world will be suspended from entering the United States for 120 days while all immigration from so-called "countries with terrorism concerns" will be suspended for 90 days.

"This is not the European way, the European Union will continue to first of all to take care and host Syrian refugees and others who are fleeing from war," EU foreign policy chief Federica Mogherini said on Monday.

The bloc was earlier angered by then President-elect Trump when he said in an interview published on Jan. 15 that Britain's exit from the EU would turn out to be "a great thing" and pledged to strike a swift trade deal with Britain.

Trump also predicted that other countries would follow Britain's lead in leaving the EU.

"I think people want...their own identity, so if you ask me...I believe others will leave," Trump said.

But Tusk said in the letter that "only together can we be fully independent."