A top European Central Bank official has defended the bank's key crisis backstop in a second day of hearings before Germany's Federal Constitutional Court.

Joerg Asmussen told the eight judges that the ECB's plan to purchase government bonds was only aimed at making sure the bank's monetary policy is effective, not at rescuing governments in the 17-country euro zone.

The ECB is prohibited by treaty from financing governments. But Asmussen insisted Wednesday that the ECB had not exceeded its powers with the bond purchases program: "It is not the goal to keep states from insolvency."

Critics argue the bond purchase program and other ECB measures exceed the bank's legal mandate and infringe on German law.

A decision by the court on the legality of the program is not expected for months.