"It remains the goal of the German government to better regulate and supervise the grey zones of the financial market," Schaeuble was quoted as saying in an advance release from an interview being published in Friday's Handelsblatt newspaper.

"We keep pushing for regulatory measures on the level of the (Group of) 20 and we remain active at the national level too. That also includes dealing consistently with the Prokon case."

Prokon, which had raised 1.4 billion euros (1.1 billion pounds) mainly from retail investors, filed for insolvency this month after a growing number of stakeholders asked for their money back following media reports about the business.

Prokon, which operates 50 wind parks in Germany and Poland and employs roughly 1,300 staff, had raised money by selling profit-participation certificates - offering high interest payments - through advertising on prime-time German television.

With profit-participation certificates, the investor also participates in losses. Unlike shares, the securities do not give holders any say in the company.

Consumer groups accused Prokon of attracting investors with promises of potential returns of at least 6 percent a year without giving sufficient warning of the risks. Its insolvency deals a blow to thousands of retail investors who had hoped to profit from Germany's shift from nuclear to renewable energy sources such as wind and solar.

Germany has long pushed for stronger financial market regulation internationally - and Schaeuble said his ministry had asked the German financial regulator Bafin to look into improving investor security.

But he also cautioned: "Even better regulation will only help up to a point if investors only look at the returns and not the risks."

(Reporting by Annika Breidthardt; Editing by Kevin Liffey)