German Chancellor Angela Merkel rejected Wednesday criticism of the ECB for not taking bolder steps to stem the euro zone debt crisis and made clear the next tranche of aid for Greece could not be paid out unless big parties in the country committed in writing to back austerity.

The European currency union is based, and this was a precondition for the creation of the union, on a central bank that has sole responsibility for monetary policy. This is its mandate. It is pursuing this. And we all need to be very careful about criticizing the European Central Bank, Merkel said in a speech to parliament.

On Greece, she added: The Greek question hasn't been cleared up yet, because the conditions are not in place for the payment of the next tranche.

For that to happen ... we need not only the signature of the Greek premier but also those of the parties that have agreed to support the government. Otherwise there can be no payout of the sixth tranche.

(Reporting by Stephen Brown; Writing by Noah Barkin)