US vice president Mike Pence has moved to allay EU fears about the strength of Washington's support for the union and its commitment to European security through the Nato military alliance.
During meetings in Brussels, Mr Pence said he was acting on behalf of President Donald Trump "to express the strong commitment of the United States to continued cooperation and partnership with the European Union".
"Whatever our differences, our two continents share the same heritage, the same values and above all the same purpose: to promote peace and prosperity through freedom, democracy and the rule of law," he said after talks with EU Council President Donald Tusk.
Mr Trump's benevolence towards Russian President Vladimir Putin and campaign rhetoric that included branding Nato obsolete and vowing to undo a series of multinational trade deals has sparked anxiety in Europe.
Mr Trump was also supportive of Britain's vote last year to leave the EU. And he has suggested that the EU itself could soon fall apart.
Mr Tusk, who chairs meetings of EU leaders, said he had been reassured after "open and frank talks" with Mr Pence, but made clear that the bloc would watch closely to ensure the US acts on its words of support.
"I heard words which are promising for the future, words which explain a lot about the new approach in Washington," Mr Tusk said.
He underlined that "too many new and sometimes surprising opinions have been voiced over this time about our relations - and our common security - for us to pretend that everything is as it used to be."
"We are counting, as always in the past, on the United States' wholehearted and unequivocal - let me repeat, unequivocal - support for the idea of a united Europe," Mr Tusk said. "The world would be a decidedly worse place if Europe were not united."
He asserted: "The idea of Nato is not obsolete, just like the values which lie at its foundation are not obsolete."
Mr Tusk added: "Both Europeans and Americans must simply practice what they preach."
After talks with Nato Secretary General Jens Stoltenberg across town, Mr Pence reiterated the administration's strong support for the alliance, but warned that Mr Trump wants to see "real progress" by the end of the year on boosting defence spending.
Nato leaders agreed in 2014 that alliance members needed to start spending at least 2% of their gross domestic product by 2024. Only five nations currently do so: the United States, Britain, Poland, Estonia and Greece.
"The truth is many others, including some of our largest allies, still lack a clear and credible path to meet this minimum goal," Mr Pence said.
Asked what the administration would do if allies failed to meet the defence spending target, Mr Pence said: "I don't know what the answer is to 'or else', but I know that the patience of the American people will not endure forever."
Mr Pence's meetings in Brussels were aimed at assuring European leaders that his words reflected the views of Mr Trump and would not easily be swept away at the whim of the US president or undermined by statements issued on Twitter.
Mr Pence, as he did in an address on Saturday at the Munich Security Conference, also said Mr Trump would demand that Russia honour its commitments to end the fighting in Ukraine.
"In the interest of peace and in the interest of innocent human lives, we hope and pray that this ceasefire takes hold," he said.
The vice president also noted the "heartbreaking" suicide bombings at the Brussels airport and subway system in March 2016, and said the US would continue to collaborate with EU partners to address safety and combat terrorism.
"The United States' commitment to the European Union is steadfast and enduring," he said.