The border dispute between Slovenia and Croatia could block further enlargement in the Western Balkans, the European Commission president warned on Monday (8 January).

"The future enlargement of the EU to Western Balkans states is in the hands of Slovenia and Croatia," Jean-Claude Juncker said ahead of a meeting with Slovenian president Borut Pahor.

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He said the deadlock between the two EU members states was "impacting the perspective of Western Balkan states to become members of the European Union."

"This is not only a bilateral problem, this is a problem that impacts the whole European Union," he said.

Croatia is refusing to implement a ruling handed down by a Permanent Court of Arbitration in The Hague last summer over the route of its 670km border with Slovenia. The two countries have been at odds over the issue since the break-up of Yugoslavia in the 1990s. Slovenia joined the EU in 2004. Croatia's accession in 2013 was, at the time, made conditional on its acceptance of international arbitration.

Croatia says that the process was tainted by links between a Slovenian arbitration judge and the Slovenian government. But the court, which dismissed the judge, said the incident did not impede its work.

Pahor went to Brussels, where he also met European Council president Donald Tusk, days after the deadline to implement the so-called arbitration award expired on 29 December.

Fishing boats

Since then, several Croatian fishing boats have entered Slovenian waters in the Bay of Piran, the most disputed area between the two countries.

Juncker, who signed Croatia's accession treaty when he was Luxembourg's prime minister, said on Monday that he will "never agree again to put off until later the resolution of [candidate countries'] border problems."

Referring to Balkan states - among which Serbia and Montenegro have already started formal accession negotiations - the commission chief added that "they have to agree between themselves before they become member states."

Juncker at the same time noted that "nobody" in the EU knows what the dispute is about and that leaders in other EU countries "have very little interest" in the issue.

His comments on the grave consequences of the dispute was a gesture to Slovenia - which has been calling for months for EU support.

Speaking alongside Juncker, Slovenian president Pahor said he wanted the commission "to become more actively involved in implementing the arbitration award."

He said the EU executive had a "political, legal, and moral duty to intervene."

"This is a question of credibility," Pahor said.

Nationalism

He added that "any reluctance" from the commission would "give a lot of room for manoeuvre to those in both countries and to those in the Western Balkans who would like … to instigate nationalism [and] breaches of international rule of law."

Juncker repeated the commission's willingness to mediate between Slovenia and Croatia but gave no detail on when or how his services could do it.

He prepared the ground for discussion by saying Slovenia and Croatia still enjoyed "friendly relations" and that the differences between them were "tiny".

"There is room for a negotiated solution," he said.

Pahor, for his part, said that Croatia could "ease the pressure" by sending a message "on the record or off the record … from a reliable source … that they are willing to respect the arbitration award."

"It would allow to enter a spirit of dialogue, a spirit of cooperation," he said.