"Our impression is that the ECB wants more information about the (cooperative and savings bank) associations to understand exactly what they are and how they work, what safety precautions they have and how liquidity sharing is organised," BVR regulatory expert Gerhard Hofmann told a news conference.

An ECB spokesman declined to comment.

The ECB has asked the BVR for information about the mutual guarantee system used by the cooperative banks and the central banks of their network, DZ Bank [DETGNY.UL] and WGZ Bank, Hofmann said.

The ECB last year conducted an intensive review of the assets and risks faced by the largest lenders in the euro area before taking over direct supervision of them in November.

Smaller lenders are still supervised primarily by national authorities but some public savings banks and cooperatives are concerned that the ECB's fresh interest in them could lead to a form of double regulation.

Germany's more than 1,000 cooperative banks are legally independent and must not be placed in the same category as cooperatives in the Netherlands or France, Hofmann said.

"We are taking precautions, because there is clearly the danger that we'll be compared with a Rabobank [RABN.UL] or a Credit Agricole," he said.

Public sector savings bank officials have said it is important to prevent European banking regulators from applying rules to small savings banks that were developed for big, globally active lenders.

(Reporting by Andreas Kroener, writing by Jonathan Gould; editing by Susan Thomas)