The European Central Bank has secretly imposed a weekly limit of about 20 billion euros ($27 billion) on its euro zone sovereign bond-buying program, according to a report in Germany's Frankfurter Allgemeine Zeitung published on Friday.

The paper, which gave no details of its sources for the story, said a limit has existed since the start of the bond-buying program in 2010, but has so far been kept secret.

Growing skepticism within the ECB about this strategy has led the central bank to reduce this limit to 20 billion euros, the newspaper said, without making clear what was the previous limit.

The ECB declined to comment on the report.

The central bank's governing council decides on the scope of bond purchases and it has previously said there is no upper limit and that the bond-buying program is temporary.

Data from the bank shows that it has only once -- when it restarted bond buying in early August -- bought more than 20 billion in any one week. Weekly purchases have been running at less than 10 billion since mid-September. ($1 = 0.740 Euros)