Brussels has asked Zagreb to provide an urgent explanation of its decision to close the border crossings with Serbia for trucks.

A spokesperson for EU foreign policy chief told a press conference on Friday that the request was sent "in the context of an investigation of the legal service of the European Commission."

The EC is looking into allegations that Croatia violated the Stabilization and Association Agreement (SAA) Serbia signed with the EU, something Serbian Prime Minister Aleksandar Vucic warned about in his letter sent to EU officials earlier in the week.

"Our first estimates show there are serious problems regarding the alignment of the decision of the Croatian government with the SAA," Maja Kocijancic said.

The spokesperson added that EU foreign policy chief Federica Mogherini again this morning spoke on the phone with Croatian Prime Minister Zoran Milanovic "and reminded him that she two days ago she publicly called on Serbia and Croatia to resolve their dispute as soon as possible in order to allow uninterrupted flow of goods."

Kocijancic said that this topic is on the agenda of EU Commissioner Johannes Hahn, who is visiting Belgrade this Friday.