Censor.NET reports citing Financial Times.
Jean-Claude Juncker said the U.S. president's enthusiasm for Britain's departure from the EU was "annoying" and "surprising". It showed the real estate tycoon was unaware of the complicated history of Europe.
The former Luxembourg prime minister said he had delivered a warning for Trump to his vice president, Mike Pence, when he visited Brussels last month.
"I told the vice-president, 'Do not say that, do not invite others to leave, because if the European Union collapses, you will have a new war in the western Balkans," Juncker said.
The official said it was important for countries in the Western Balkans to have the prospect of EU membership, "If we leave them alone - Bosnia Herzegovina, Republika Srpska, Macedonia, Albania, all those countries - we will have war again."