BRUSSELS/LJUBLJANA (Reuters) - An adviser at the EU Court of Justice agreed with Croatia on Wednesday that settling the border between Slovenia and Croatia was not a matter on which EU courts should rule.

Slovenia had argued that, by not implementing an arbitration tribunal’s determination in 2017 of the land and sea border between Croatia and Slovenia, Croatia was violating EU law and could be sued under EU law.

“Determining that boundary is, by its very nature, a matter of public international law in respect of which the court does not have jurisdiction,” Advocate General Priit Pikamae said in his non-binding opinion on Wednesday.

The dispute risks complicating Croatia’s accession to the Schengen border-free area. The European Commission has said Croatia met technical criteria for accession, but Slovenia is unhappy with Croatia’s rejection of the arbitration ruling, which might affect Slovenia’s stance on Croatia’s Schengen hopes.

The problem also affects fishermen of both countries in the zone where the maritime border has not yet been agreed upon.

Slovenian Foreign Minister Miro Cerar told a news conference later on Wednesday that he believed the court would not follow the adviser’s guidance, adding the opinion and consequent court decision “has no influence on the validity of the arbitration ruling” regarding the border.

Slovenian Prime Minister Marjan Sarec told reporters Slovenia remains adamant that the arbitration ruling on the border between the two states has to be implemented.

Croatian Prime Minister Andrej Plenkovic welcomed the adviser’s opinion and invited Slovenia on a dialogue to resolve the problem.

“We are friendly countries and we’re ready to seek in a dialogue a solution which would satisfy both countries. I expect some moves in that direction in the coming months,” Plenkovic said at the beginning of a cabinet session in Zagreb.

Zagreb does not want to implement the arbitration ruling saying Slovenia improperly meddled with the arbitration process.

The court itself will likely rule in two to four months on the matter. It typically follows the opinions of its advocate generals.