The Fine Gael leader, Mr John Bruton, said yesterday he did not believe that NATO was acceptable as the body to provide Europe's defence capacity.
During an online question-and-answer session as part of the European election coverage of The Irish Times on the Web, Mr Bruton said: "NATO is not under European control, and its military operations do not seem to have proper political accountability."
He went on to criticise aspects of NATO's Balkans campaign. "Some of the choices of targets in Serbia made by NATO are ones that would not have received political approval in Europe in any circumstances," he said, citing the destruction of Belgrade's central heating system as one.
Speaking on the same issue the previous day, the Labour leader, Mr Ruairi Quinn, said the European Union had to develop a comprehensive security and defence capability.
"The crisis in the Balkans has demonstrated the necessity for that facility. My own view is that within the framework of the United Nations a new Northern Hemisphere defence union should be established to replace NATO.
"This new union would have three pillars - North America, European Union, Russia and the rest of Europe. NATO is a legacy from the Cold War and its presence in a formal sense in some parts of central Europe is still resisted psychologically and politically," he said.
He then called for a widespread debate in Ireland and across Europe about what he termed "the type of security structures that are relevant to the 21st century." Mr Quinn went on to say: "This will result in a treaty-based structure, regulated by international law and accountable to the European and national parliaments. Such accountability is decidedly absent from NATO's actions at present."
The Sinn Fein president, Mr Gerry Adams, will answer readers' questions from 12.30 p.m. today and Mr Trevor Sargent of the Green Party will be there today at 3 p.m.