Euro zone leaders agreed in July to guarantee more lending to Greece to allow the debt-laden country to shore up its public finances and restart the economy and avoid a default.

But Finland's parliament had decided that the country would have to get collateral for any new loan guarantees it makes, irking many euro zone countries, the International Monetary Fund and the European Commission because such a demand further complicated the already complex new financing plan for Athens.

The chairman of euro zone finance ministers, Jean-Claude Juncker, said last week that he expected a deal on the collateral at the informal meeting of euro zone and European Union ministers, called the Ecofin, on Sept. 16 in Poland.

But euro zone officials said after talks to prepare that meeting, that a definitive solution could take longer.

"I think it will take more time than until the informal Ecofin -- it is very complicated technically," one euro zone official involved in the preparations said. "The end of the month at the earliest I think -- the problems are mainly of a legal nature, they are very, very complicated legal issues."

"There is no bad will on anybody's part. Greece is cooperating in a very forthcoming manner on this. It will be a solution that in principle should be open to everyone, but that very few countries will want to use it, because in exchange they will have to forego certain other privileges," the official said.

Slovakia, Austria and the Netherlands have also expressed interest in getting collateral from Greece for their loan guarantees, but euro zone officials are keen to limit the use of such collateral to Finland, which is the only country where parliament had made such a demand.

There is pressure to find a solution because without one, Greece will not be able to get the new financing package, which would lead to a default from next year.

"There will be quite a bit of political pressure to solve it on the political level on September 16, but then technical work could continue," a second euro zone official said.

"It will not be 100 percent ready in the coming days."