Addressing the leadership of the state union and its two republics at a ceremony in Belgrade, the EU official said the opening of the negotiations marked the start of a new stage in relations between the EU and Serbia and Montenegro.
Rehn added that the negotiations would be held on two tracks, separately with negotiators of the two member-states and with negotiators of the state union on matters of common interest.
In this context Rehn stressed the importance of full cooperation with the Hague-based International Criminal Tribunal for the former Yugoslavia (ICTY).
It is most important to ensure full cooperation, as the European Commission will not hesitate to propose the suspension of the negotiations if the cooperation comes to a halt, Rehn said, adding that it was not good to waste time on issues such as the outcome of a referendum on Montenegro's independence, which is likely to take place next spring.
The President of Serbia-Montenegro, Svetozar Marovic, described today's opening of the negotiations as "a great day for Serbia-Montenegro".
Marovic added that the destiny of the state was more important than the destiny of individual indictees accused of war crimes. In this context Marovic called for solving the case of war crimes indictee Ratko Mladic as soon as possible.
Prime Ministers Vojislav Kostunica of Serbia and Milo Djukanovic of Montenegro said that negotiating teams of the two states were fully prepared for the start of the negotiations.
David Gowen, the Ambassador of Great Britain which is the current chair of the European bloc, welcomed the start of the negotiations and conveyed his country's readiness to help Serbia-Montenegro on its European path.