Formal exit talks with the 27 other members of the EU were due to start on June 19. The parties must reach an agreement by March 2019, if the UK is to avoid crashing out of the bloc with no deal.
"We don't know when Brexit talks start," tweeted European Council President Donald Tusk Friday as the election results became clear. "We know when they must end. Do your best to avoid a 'no deal' as a result of 'no negotiations,'" he advised.
Others, too, called on UK Prime Minister Theresa May to hurry up and resolve the issue.
Jean-Claude Juncker, President of the European Commission, said he was "waiting for visitors coming from London," that he was ready to "open negotiations tomorrow morning at 9.30."
Juncker said he hoped there would be no further delays, and that the result of the UK general election would not have a "major impact on the negotiations we are desperately waiting for."
"We are ready. Michel Barnier (the EU's chief Brexit negotiator) is ready," German EU Commissioner Günther Oettinger, who is responsible for the EU budget, told a German radio show. "We are working towards it."
"We will negotiate strongly but fairly," he insisted, "but whether the other negotiation partner can even negotiate, we must wait and see in the next few hours, the next few days."
"Two strong partners are confident, work faster and get to results they can both accept. A weak partner weakens the whole negotiations," he added.
Barnier indicated on Twitter that the EU was prepared to show some patience, making clear that talks should not begin until the UK's domestic political chaos is resolved.
But the European Union's foreign policy chief, Federica Mogherini, was less understanding.
"One year after their referendum, we still don't know the British position in the negotiations on Brexit and it seems difficult to predict when we will, because democracy often requires time," she said.
Swipes at Theresa May
Elected EU officials and members of the European Parliament aimed their fire squarely at Theresa May -- who called the snap election in an attempt to strengthen her hand in the Brexit talks -- and the wider Conservative Party.
"Yet another own goal, after Cameron, now May, will make already complex negotiations even more complicated," tweeted Guy Verhofstadt, the European Parliament's lead Brexit negotiator, referring to former PM David Cameron's decision to call a referendum on the EU, which he then lost.
Verhofstadt, the former Prime Minister of Belgium, added that if the Conservative Party focuses on attacking Brussels and the EU, "they will just get deeper in self-inflicted trouble. Time for action, not words."
"EU did not want #Brexit, but has been prepared to negotiate it since last year," tweeted Siegfried Muresan, spokesperson for the European Parliament's largest grouping, the center-right European People's Party (EPP). "UK wanted it, but is still not ready to negotiate."
He then posted "Fact is, UK has no good options. Since 24 June 2016 (the date of EU referendum) Unfortunate."
The Conservative Party left the EPP in 2009 amid concerns about accelerated EU political integration.
There was a more conciliatory tone towards May from France.
"The British have spoken, they have voted, and have given the Conservative party a majority, albeit a simple majority, which is something of a surprise," Prime Minister Edouard Philippe said on France's Europe 1 radio station.
"I don't think we should read these results as calling into question the stance on Brexit which was clearly expressed by the British people," he added.
But others noted that the UK political landscape has never looked less clear. "Could be mess for the United Kingdom in the years ahead," tweeted veteran Swedish politician and co-chair of the European Council on Foreign Relations Carl Bildt as the first results came through.
"One mess risks following another," he added. "Price to be paid for lack of true leadership."