European Union top diplomat Javier Solana voiced Wednesday opposition to formally establishing the outer borders for the bloc.
"The borders cannot be set in stone. We should not finalize them," Solana told German magazine Die Zeit.
But the EU, he said, could decide "not to expand much further than the circle of countries with whom we have already started negotiations or have pledged a European perspective to, such as the Balkan states."
EU member states are divided over how far the bloc should expand and how to proceed with the stalled constitution.
Germany hopes to set up a road map to go ahead with a revised version of the treaty before its presidency expire in June, but Czech President Vaclav Klaus opposes the plan.
German Chancellor Angela Merkel will meet him on April 17 near Berlin to discuss the issue.