Polish prosecutors have opened an investigation into the head of the country's Constitutional Tribunal to determine if he abused his power in not allowing judges appointed by the ruling party to take part in rulings.

The investigation into Andrzej Rzeplinski, which opened Thursday, is the latest development in an ongoing conflict between the Polish government and the constitutional court, whose role is similar to the U.S. Supreme Court.

The government's conflict with the court has raised international concerns about the state of democracy in Poland, and critics have slammed the investigation into Rzeplinski as an attack on the separation of powers.

Rzeplinski called the investigation part of a "hybrid war" against the tribunal and "a feeble attempt to interfere in the independence and autonomy of the judiciary."