Private creditors should contribute to a new Greek bailout starting immediately, the head of the Bavarian wing of German Chancellor Angela Merkel's conservatives told magazine Der Spiegel.

In comments to be published on Sunday, Horst Seehofer, the outspoken head of the Christian Social Union (CSU), took a stance at odds with an outline agreement made between Merkel and French President Nicolas Sarkozy on Friday -- a deal that went some way toward calming financial market fears of a Greek default.

But Seehofer said: Experts have been telling me for a year that a Greek restructuring is necessary. Now is the time for private creditors to start to contribute.

Merkel and Sarkozy had said they agreed on a mild solution favored by Paris and the European Central Bank, in a move to resolve differences over how to involve private holders of Greek bonds in a new rescue package for Athens.

Merkel called the softer plan -- based on the 2009 Vienna Initiative by banks to voluntarily maintain exposures in Eastern Europe at the height of the financial crisis -- a good foundation for a Greek deal.

Most economists are highly skeptical that Greece can ever repay its debt mountain, which has reached 340 billion euros ($480 billion) or 150 percent of the country's annual economic output.

In Der Spiegel, other parliamentarians from Merkel's ruling coalition also took aim at her Greece policy, saying that participation from private creditors should be imposed.

We need a haircut, and it will not be voluntary, finance expert Manfred Kolbe from Merkel's Christian Democrats told the magazine. CSU Europe expert Thomas Silberhorn called for a compulsory participation.

Binding rules with a compulsory participation of private creditors, are needed, Silberhorn said.

(Writing by Brian Rohan; Editing by Ruth Pitchford)