He was speaking at the two-day "Croatia Summit 2007 - Europe's New South", which began in the southern Adriatic resort today.
Scheffer said that UN mediator Martti Ahtisaari's plan for Kosovo was a good basis and underlined that the situation in Kosovo could not and must not be changed by force.
Without mentioning dates or deadlines, he reiterated that NATO's doors were open to new members, but said that every country must invest additional efforts to cross the doorstep.
Scheffer commended Croatia for its engagement in NATO's international missions and for winning public support for membership of the alliance. He underlined that like all the countries in the region, Croatia too must continue with political and military reforms.
Speaking of Serbia, Montenegro, and Bosnia and Herzegovina, the secretary-general said their participation in Partnership for Peace offered them a chance to join the rest of the region, but only if they looked to the future and not to the past.
US Assistant Secretary of State Dan Fried read out a note from President George W. Bush, who said that the United States would continue to support the process of reconciliation and progress in Southeast Europe.
America will firmly stand by those who defend democracy. Your efforts in building a brighter future give hope to everybody who wants to live in peace and freedom, Bush said in the note.
The summit's host, Croatian Prime Minister Ivo Sanader, underlined that Croatia would soon join NATO and the European Union, saying this increased its responsibility to security issues and the immediate region where Croatia would be constructive, cooperative and exemplary.
Sanader expects an open debate on global security and on stability, development and economic growth in Southeast Europe.
The summit gathered presidents of state or prime ministers from Croatia, Romania, Latvia, Moldova, Slovakia, Georgia and Kosovo as well as numerous officials from international organisations. Another 40 countries are represented at various political levels. Serbia has no representative, while Albanian PM Sali Berisha cancelled his attendance due to the election of a new parliament speaker.