Cerar, who hosted Luxembourg counterpart Xavier Bettel, Dutch Prime Minister Mark Rutte and Belgian Foreign Minister Didier Reynders for a Slovenia-Benelux summit in Ljubljana, said the "arbitration decision is final and binding on both countries".

"It is an international legal obligation for Slovenia and Croatia to implement it," he added, stressing the decision is also incorporated in EU law, meaning Croatia is violating EU law by refusing to implement it.

"If the Croatian position does not change fast, Slovenia will be forced to resort to legal means," said Cerar, whose view was also backed by his three guests.

Cerar was responding to statements made earlier in the day in the European Parliament, including by Juncker, who said "this is a bilateral issue but also an EU issue that needs to be resolved urgently".

Juncker moreover reiterated his assurance that there would be no more EU accessions by any Western Balkan country without a prior resolution of all potential border issues for the candidate country.

The border issue was broached as the European Parliament discussed the EU's future with Croatian President Andrej Plenković. The latter had not mentioned it in his address, he however later responded to comments made during the discussion.

Plenković reiterated Croatia had withdrawn from arbitration for a reason and again called for a bilateral search for a solution "that would benefit both countries".

He urged against the issue being put on the shoulders of fishermen, who are being issued fines by both countries as a result of a different interpretation of the course of the border.

A call to Croatia to respect the June 2017 arbitration decision meanwhile came from Austrian SocDems MEP Josef Weidenholzer, while ALDE leader Guy Verhofstadt also called for a solution based on the arbitration award.

Slovenian MEPs repeated Slovenia's main arguments, with Igor Šoltes (Greens) stressing the rule of law is a cornerstone of European values and needed to be respected and Tanja Fajon (S&D/SD) wondering what kind of message Croatia is sending to other countries in the Western Balkans wishing to join the EU.

Ivo Vajgl (Alde/DeSUS) rejected Croatia's reasons for withdrawing from the arbitration process, stressing the arbitration tribunal had ruled there had been no violations in the arbitration process that would warrant that.

Lojze Peterle (EPP/NSi) rejected Plenković's call against dramatising the situation, saying no such thing was happening. "We are no longer looking for a solution because the arbitration tribunal already found a solution. What is necessary now is its implementation," he said.