US vice president Mike Pence has reassured European Union officials about the Trump administration's stance towards the EU, according to European Council president Donald Tusk.
Mr Tusk said he and Mr Pence held "open and frank talks" aimed in part at allaying concerns about President Donald Trump's support of the EU and of European security through the Nato military alliance.
"I heard words which are promising for the future, words which explain a lot about the new approach in Washington," Mr Tusk said.
Mr Tusk said that "too many new and sometimes surprising opinions have been voiced... for us to pretend that everything is as it used to be".
Mr Pence said he was looking to explore ways to "deepen our relationship with the European Union and the European community" as he opened a day of meetings on the EU and Nato with EU foreign policy chief Federica Mogherini in Brussels.
Mr Pence was facing deep scepticism among European leaders over the foreign policy direction of his boss.
Mr Trump was supportive of Britain's vote last year to leave the EU, and he has suggested that the EU itself could soon fall apart.
Mr Pence voiced the administration's strong support for Nato over the weekend.
Mr Pence said he was acting on behalf of Mr Trump "to express the strong commitment of the United States to continued cooperation and partnership with the European Union."
He added: "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."
Mr Trump's election campaign rhetoric - branding Nato obsolete and vowing to undo a series of multinational trade deals - and his benevolence towards Russian President Vladimir Putin sparked anxiety across the Atlantic.
Mr Tusk said: "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. The world would be a decidedly worse place if Europe were not united."
He added: "The idea of Nato is not obsolete, just like the values which lie at its foundation are not obsolete."