European Union President Donald Tusk says China, Russia and the U.S. administration of President Donald Trump are among the top external threats facing the bloc.

His remarks came in a letter to 27 EU leaders before a summit Friday in Malta.

"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 said.

Trump has insisted on policies that put the United States first, and has questioned whether NATO members are contributing their fair share to the alliance.

Tusk laid out a defense of the European Union, saying it has never faced more dangerous challenges and needs "courage, determination and political solidarity."

He said the European Union must stand up for its dignity in talks with the United States, Russia, China or Turkey, and must not give in to populist arguments and xenophobic sentiments that go against European integration.

"It must be made crystal clear that 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. Only together can we be fully independent," Tusk wrote.

With Trump calling for renegotiated trade deals, Tusk advocated for the European Union to protect its citizens and businesses and remember "that free trade means fair trade."

He ended the letter by saying the bloc cannot give in to those who want to "weaken or invalidate" the bond between the United States and the European Union.

Tusk wrote, "We should remind our American friends of their own motto: 'United we stand, divided we fall."

The U.N. human rights office criticized Trump’s travel restrictions Wednesday, calling the executive order a “breach of Washington’s human rights obligations.

In a statement signed by several U.N. human rights officials, the organization said the presidential directive was “deeply troubling” and could lead to further violence against people fleeing war zones.

“The U.S. recent policy on immigration also risks people being returned, without proper individual assessments and asylum procedures, to places in which they risk being subjected to torture and other cruel, inhuman or degrading treatment, in direct contravention of international humanitarian and human rights laws which uphold the principle of non-refoulement,” the statement said.

The U.N. officials said the United States has a responsibility to accept refugees because the country is involved in conflicts that are forcing refugees to flee their homes in Iraq and Syria.

A top Vatican official also expressed concern over Trump’s executive order Wednesday and called on Americans to “integrate those who arrive, who come into our society, into our culture."

"We are builders of bridges, far less of walls, and all Christians should emphatically reaffirm this message,” Monsignor Angelo Becciu, told the Catholic TV channel TV2000.