BERLIN - Germany is considering whether it should tighten the reporting requirements for investors who are building up stakes in the nation's companies, the finance ministry said on Monday.

The move comes after Italy's UniCredit recently acquired a hefty stake in Germany's Commerzbank that took the government by surprise.

Germany is examining whether it could require investors to disclose a 3% stake that comprises a combination of shares and derivatives.

The current rule is disclosure of such combined holdings at the 5% level.

UniCredit had quietly built a roughly 4.5% stake in Commerzbank through shares and derivatives that it had not been required to disclose before buying an additional 4.5% stake of the bank in September from the German government, in a purchase that caught Berlin off-guard.

The development was first reported by German newsletter Finanz-Szene.

(Reporting by Christian Kraemer, writing by Tom Sims, editing by Thomas Seythal and Matthias Williams)