The European Central Bank alone cannot solve the causes of the euro zone's problems, board member Joerg Asmussen has said, reiterating that member state governments must pursue structural reforms.

"The structural causes (of the euro zone crisis) cannot be solved with the instruments we have,'' Asmussen said on European radio.

''Euro zone member states must keep up the momentum with vigour and patience, I know sometimes it is a painful process, but they must not ease up on their efforts to fight the causes of the crisis," he added.

He also reiterated the view he presented this week to the German Constitutional Court, saying the ECB bond-buying programme does not violate the bank's mandate.

The court is considering the legality of the ECB bond-buying scheme under German law.