Dimitris Avramopoulos, Commissioner for Migration, Home Affairs and Citizenship, warned the rising tides of nationalism and populism in the Balkan states, could plunge the region into chaos.
And he said Germany had shown the world what could be done with a new outlook focused on international cooperation and humanitarianism.
He said: “It is important that we do not return to the dark past.
“The leadership of these countries comprehend it is vital to think and act in a European spirit.”
Mr Avramopoulos said, as a neighbouring country, Greece’s role in keeping the Balkans in check was crucial.
He claimed countries such as Greece could “build bridges” that will bring the West Balkans closer to the European family.
And he said: ”Germany has shown Europe and the whole world what can be done by a country based on humanitarian and democratic values.”
The EU is keen for Serbia and Montenegro to join the bloc next, possibly by 2025.
And it has tried to speed-up steps to bring other western Balkans states into the fold.
Albania, Bosnia, Kosovo, Macedonia, Montenegro and Serbia all have hopes to join the EU.
But there has been rising euroscepticism at home, particularly around migration issues.
And the region is still scarred by the ethnic conflicts that followed the breakup of Yugoslavia in the 1990s.
Bosnia remains deeply divided while Serbia does not recognise the independence of Kosovo, its former province, accusing it of seeking war with Belgrade.
Montenegro, meanwhile was mired in political crisis for much of 2017.
The EU’s strategy rests on Serbia and Kosovo reaching a “normalisation of relations” by the end of 2019.
But Western leaders have accused Russia, traditional ally of the Serbs, of seeking to exploit diminishing EU leverage in the Balkans by manipulating political events in the region.
(Additional reporting by Maria Ortega.)