A surrender of sovereignty by countries in the euro zone to tighten fiscal integration in the bloc may only be possible in the medium term.

This is according to the EU's economic and monetary affairs commissioner, Olli Rehn.

The European Central Bank and others have called for closer fiscal integration among the 17 countries that use the euro.

This process will involve widening the powers of the European Commission in controlling national budgets.

Olli Rehn's warning that it will take time for countries to give up such national autonomy highlights the obstacles to advancing rapidly towards such a goal.

"More fiscal integration would likely involve more explicit transfers of sovereignty," Olli Rehn said in the text of a speech delivered at the European Parliament in Strasbourg. "This may be possible only in the medium term."

EU leaders will discuss tighter cooperation at a summit in Brussels at the end of June aimed at underscoring their commitment to the euro and convincing investors of the currency's future. Any deeper integration could be a step towards the issuance of common euro zone debt, but that is in reality still years away.

Rehn said that clarifying the steps that would be taken towards integration would rebuild confidence. He also called for movement towards a banking union, steps which he said could be taken more quickly.

The term banking union refers to strengthening supervision of large lenders in Europe as well as establishing a single bank resolution authority and deposit guarantee scheme.

"Already in the short term, more can be done to strengthen the financial sector in Europe to establish a financial union to complement the economic and monetary union," Rehn, the EU's top economic official, said.

The European Commission already has significant powers in monitoring national budgets. Under existing rules, it is possible to fine countries that miss budget targets.

It is down to the European Commission to propose sanctions, first by demanding that countries hand over a deposit of money and later by requiring them to pay a fine.

Draft legislation, known as the two-pack, will extend the powers of the EU's executive in monitoring countries spending by granting the Commission a say over national budgets before they are finalised.