The Belgian government says it will quiz Luxembourg over media allegations that it makes unwritten tax agreements with companies despite European Union insistence to promote full transparency on the much-criticized practice.

The EU has been trying to crack down on preferential tax treatment for big multinationals in member states and has said that taxation deals must be as public as possible.

Belgian Finance Minister Johan Van Overtveldt said Tuesday that he would "certainly ask my Luxembourg colleague to shed some light on this. If this is true — the "if" is very important — it would be fully counter to the where we need to go, namely more transparency."

Van Overtveldt will be looking to talk to his Luxembourg counterpart, Pierre Gramegna, during two days of EU talks starting Tuesday.