The US Vice-President, Mike Pence, has said the US under President Trump remains committed to co-operation and partnership with the European Union.
Mr Pence, speaking in Brussels, sought to reassure European allies worried by some of Mr Trump’s statements.
Donald Tusk, the president of the European Council, said the EU was counting on unequivocal US support for a united Europe.
Mr Trump’s comments on Brexit and the role of Nato have unsettled allies.
Why are EU leaders worried?
Donald Trump has been vocal in his criticism of international organisations, including the North Atlantic Treaty Organisation (Nato), a defence alliance which was initially formed to meet the military threat to Europe from the Soviet Union.
Mr Trump has described Nato as “obsolete”, and criticised members for not meeting the target of spending 2% of their GDP on defence.
He is also seen as friendlier to Russia than his predecessors, and some of his former aides have come under scrutiny for their alleged links with Russia.
And he has spoken out in favour of Brexit, telling the Times newspaper last month that he thought the UK was “so smart in getting out”, and predicting that “others will leave” the EU.
European Commission President Jean-Claude Juncker has said the US needs “a strong, united European Union”.
“This is not the moment for Europe to divide itself into former national, provincial categories,” he said.
Has Mike Pence managed to reassure EU leaders?
The vice-president says it was Mr Trump who asked him to travel to Brussels for talks with Mr Tusk and Mr Juncker.
Mr Pence told leaders gathered at the Munich Security Conference at the weekend that the US would be “unwavering” in its support for Nato.
He also said the US would continue to hold Russia accountable over the Ukraine conflict, though he said that President Trump believed that new common ground could be found with Russia.
He said the US was committed to “partnership with the European Union”, stressed that they “shared the same heritage [and] the same values”.
Mr Tusk said the meeting with Mr Pence was “truly needed”, after what he described as “too many new and sometimes surprising opinions” that had been voiced in past months “for us to pretend that everything is as it used to be”.
Mr Tusk also said Mr Pence had confirmed the US supported “the idea of a united Europe”.
EU officials told reporters they were encouraged by the meeting, with one saying “we got everything we were looking for”.
However, correspondents say EU leaders will also be watching Mr Trump’s words, and actions, to see if they match what his vice-president has pledged.
Analysis by Damian Grammaticas, BBC Europe correspondent
For European leaders this was reassurance, of a sort.
When the US vice-president arrived in Europe at the weekend and didn’t mention US backing for the EU it was noted. Now Mr Pence has done so, importantly in the home of the EU’s institutions, standing side by side with Mr Tusk and Mr Juncker.
But the fact Europeans were waiting for a US leader to make such a statement is a sign these are uncertain times. And it leaves a nagging feeling among Europeans that they can’t be sure President Trump will stick to the same script or differ in his next tweet or unscripted remark.
So for many Europeans the Trump administration remains unpredictable, unclear in its thinking, even divided on key issues.
And Europeans may well wait, putting more store by what a transactional president does, than by what is said. Mr Tusk made a point of telling Vice-President Pence that “Americans and Europeans must… practise what they preach.”