European Central Bank head Mario Draghi is defending the ECB's bond-buying program, saying it is more important now amid recent market uncertainty.

Draghi said Tuesday in the text of a speech in Berlin that the offer made last summer to buy bonds in countries struggling with high borrowing costs "is even more essential now as we see potential changes in the monetary policy stance, with associated uncertainty, in other jurisdictions of the global economy."

The U.S. Federal Reserve has roiled markets by indicating it could taper off its emergency stimulus measures next year.

The ECB hasn't bought any bonds, but the offer has lowered borrowing costs for indebted governments. The plan is being challenged in Germany's Federal Constitutional Court and was opposed by Germany's central bank.