He also, however, said that he did not think it likely governments would be persuaded to contribute more if Greece's deal making private sector investors take a hit on their holdings of Greek debt fall through.

Finnish objections to bailouts of Greece and others has proved a stumbling block for EU policymakers over the past two years, but been overcome by a combination of domestic political dealing and exceptions being made at a European level.

The European Stability Mechanism, scheduled to start operation in July, plans to have decisions made by qualified majority rather than unanimity to avoid changes being stalled by one dissenting member country.

Finland's parliamentary committee deemed the change unconstitutional, and politicians fear it would harm interests of smaller countries like Finland.

"I am very confident that such a solution will be found that fits everybody," Katainen told Reuters in an interview.

"It is impossible to think Finland would be outside the ESM," he said, adding senior Finnish officials were seeking a solution.

"It will enable efficient decision-making in the ESM and it will be possible from our national decision-making perspective," he said, declining to elaborate.

Katainen and his National Coalition party are pro-euro, but policymakers have taken a critical stance on some EU issues given voters' dissatisfaction over European bailouts.

Reuters reported earlier that Greek talks with private sector creditors over part of a second bailout were going badly, raising prospects that euro zone governments must contribute more to the aid package.

Katainen said he did not believe member states would pay more.

"Until everything has been done on the debt restructuring side there is no use to talk about increasing member states' liabilities," he said.

"At this point I don't see it realistic that the contributions would be raised," he said.

Katainen also echoed the view of German and Italian leaders by saying he wants a financial transaction tax to be applied to the entire Europe Union, not just the 17 euro zone countries.

"It should cover the whole of Europe. From the Finnish point of view, if Sweden is outside, it would have a negative impact for the development of Finland's financial markets and economy."

Among the 10 EU countries outside the euro zone, Britain and Sweden are against the tax, fearing it would harm their financial sectors. France is keen to push ahead with such a "Tobin tax" even without all of its EU partners.

(Reporting By Jussi Rosendahl; editing by Patrick Graham)

By Jussi Rosendahl