A plan to extend Greek bond maturities with the support of private sector creditors gained momentum Wednesday as the German finance ministry, banking sources and a major financial industry lobby said they were initiating talks on how to carry out a voluntary rollover on Greek debt.

The German Finance Ministry will host a meeting with major German banks and insurers like Deutsche Bank AG (DB), Commerzbank AG (CBK.XE) and Allianz SE (AZ) Wednesday to discuss how a voluntary debt rollover by bondholders could shape up, several banking sources familiar with the matter said, adding that a plan wouldn't be finalized Wednesday.

German banks have the highest exposure to Greek sovereign debt, worth around $22.7 billion, according to figures released in early June by the Bank of International Settlements.

A spokesman for the German finance ministry wouldn't comment on the reported meeting, but said the ministry is "trying hard to get into talks with the private sector on a national and international level to see how we can quantify participation," before a July 3 meeting between euro zone finance ministers.

The Dutch government has started similar discussions with local financial institutions, another person familiar with the matter said Wednesday.

Euro-zone politicians want Greek government debt bondholders to make a contribution to a new funding package for the country by using the money they receive from maturing bonds to buy new bonds on roughly the same terms.

Securing private sector support for a rollover has emerged as a vital next step in alleviating Greece's immediate debt woes, after Germany and France agreed last week to involve private sector creditors in further aid measures and Greece's parliament signed off late Tuesday on a new round of budget cuts.

The Institute of International Finance, a lobby representing more than 400 financial institutions around the world, has also entered discussions with the Greek government, other international public authorities and its own membership about Greece's debt problems, but only on an "informal" basis, according to a spokesman.

German daily Handelsblatt Tuesday reported that euro-zone policy makers had asked the IIF to review possibilities for a rollover of Greece's sovereign bonds.

The IIF stressed Wednesday that the discussions to date haven't involved a formal, policy-making role for the trade group, although IIF managing director Charles Dallara is currently in Greece to assist with the situation.

Roughly half of the IIF's members are based in Europe, including a number of institutions that have significant holdings of Greek government bonds. The IIF is chaired by Deutsche Bank AG's (DB) Chief Executive Josef Ackermann.

Deutsche Bank, which is Germany's largest listed lender, had a total net exposure of EUR1.6 billion at the end of 2010.

Major European banks, insurers and institutional investors are also holding talks with credit ratings agencies, who could still derail the plan if they decide it qualifies as a default scenario.

A so-called "credit event", or default, declared by the ratings agencies would cut Greek banks off from vital European Central Bank funding and spark a local banking crisis with spillover effect for broader financial markets.

Fitch Ratings warned again Tuesday that even a voluntary extension on debt maturities could qualify as a default scenario for Greece.

But economists and bank analysts expect its still possible to reach an agreement with the ratings agencies that avoids triggering a default. "We assume that there is some room for manoeuvre for a compromise," BNP Paribas economists wrote Wednesday in a research report.

By William Launder, Dow Jones Newswires; +49(0)6929725515; william.launder@dowjones.com

(Paul Hannon, Eyk Henning, Joern Rehren, Bernd Radowitz and Anna van der Meulen contributed to this article.)