Slovenia and Croatia should implement the border arbitration ruling, but the responsibility for this lays with the two countries, Timmermans said in an interview with Delo's Saturday's supplement.

"We hope both sides will honour the ruling and implement it," he reiterated.

Timmermans praised the Slovenian and Croatian prime ministers for maintaining dialogue. "I know it's not easy. But we're sticking with the position that the arbitration has been carried out and that both sides had agreed on the process," he said.

Commenting on the 2015 wiretapping scandal, revealing contacts between the Slovenian arbitrator and agent, which is Croatia's argument for not honouring the arbitration ruling, Timmermans said the contacts between the arbitrator and agent had been a "big mistake" but that this had subsequently been settled in line with the rules of the arbitration.

"It was clear that the arbitration could continue. That is how we see it too," he said about the Commission's view.

"That's important for your bilateral relations, but also for the EU. It's important that such issues between member states do not remain unsolved if you decide for arbitration."

The Commission has made its position on this very clear, he said. "We have the arbitration ruling. We do not need to assess it - whether it is good or not. It would be best for both sides to implement it."

Asked what happens if one side says it will not implement it, Timmermans replied that "we're not there yet".

When asked whether he expected Croatia, which has been rejecting the ruling, to change its mind, he said "we shall see".

The point is that this is a bilateral issue, that the Commission has always supported the arbitration and that the Commission thinks both sides should implement the ruling, he said, adding that the Commission would stick to this position.

The Permanent Court of Arbitration in The Hague declared its decision on the course of sea and land border between Slovenia and Croatia on 29 June. Slovenia said it would implement the ruling, while Croatia insists that the decision is not binding for the country because it had unilaterally withdrawn from the process.