European Union (EU) chief Donald Tusk says that the first weeks of Donald Trump as US president are contributing to the "highly unpredictable" outlook for the bloc.
In a letter to 27 leaders, EU council president Mr Tusk mentioned the Trump administration as part of an external "threat" together with China, Russia, radical Islam, war and terror.
The letter was not sent to the UK, which is poised to leave the bloc.
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".
Mr 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," Mr 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 was not 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 Mr Trump did not endear himself with many EU leaders by saying that Brexit "will be a tremendous asset and not a tremendous liability".
Mr 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".
He further insisted any disintegration would not 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".