European Union leaders toughened their tone towards aspiring new members on Thursday, at the start of a summit designed to reassure wary voters while keeping the door ajar to countries queuing to join, according to Reuters. The two-day summit comes after the 25-member bloc agreed on Monday to a partial freeze in Turkey's entry talks over its failure to normalise trade with Cyprus. The move papered over internal rifts over how to treat the mainly Muslim country, but French conservative presidential frontrunner Nicolas Sarkozy served notice he could block all membership negotiations with Turkey if elected. "You know my reticence on the subject. And I'm happy to see that these ideas are gaining ground," Sarkozy said after a meeting of EU conservative leaders just before the summit. A German participant said Sarkozy had told them privately that if elected he would have an obligation to the French people -- an apparent hint at a veto -- and urged them to start working on an alternative "privileged partnership" with Ankara. The summit leaders were set to reaffirm backing for the eventual membership of Turkey, and the western Balkan states -- Albania, Bosnia, Croatia, Macedonia, Montenegro and Serbia -- but to agree on a de facto slowing of the enlargement process. While welcoming Bulgaria and Romania to the bloc from Jan. 1, they will insist any further enlargement must await reform of the EU's ailing institutions -- a process Germany aims to revive when it succeeds Finland as rotating president next month. "The important issue is the problem if ... we want to go on with further enlargement without having arranged the institutional settings, or if we want to have our house in order before accepting new inhabitants," Luxembourg Prime Minister Jean-Claude Juncker said on arrival. THREAT OR INCENTIVE? German Chancellor Angela Merkel stressed the EU had to be tough in ensuring new entrants fully met membership criteria. "I am not saying this as a threat but as an incentive for countries who want to join and for the community," Merkel told the German parliament before heading for Brussels. Sarkozy, who did not attend the EU summit, said he felt growing support for his vision of a "Europe with borders". But Swedish Prime Minister Fredrik Reinfeldt insisted the summit would "not close doors to anyone". "We are not writing maps of Europe, saying here is the border where it ends or something like that," he told Reuters. Qualms over Turkey's EU bid were a factor in last year's "No" votes in France and the Netherlands to a constitution designed to revamp EU structures to welcome in further members after Romania and Bulgaria. Romania and Bulgaria celebrated their entry before the summit. Romanian President Traian Basescu, Bulgarian Prime Minister Sergei Stanishev and EU Commission President Jose Manuel Barroso were joined by flag-waving local children at a Brussels church. The accession of the two states will complete a "big bang" expansion that began in 2004 when the EU admitted 10 mostly ex-communist central and eastern European countries that has since prompted a public backlash of "enlargement fatigue". Participants quoted Austrian Chancellor Wolfgang Schuessel as telling the conservative leaders he believed Turkey would eventually get a status short of full membership. Turkey's backers, including Britain and Sweden, see the country as a vital link with the Muslim world and are playing down the impact on accession talks seen spanning over a decade. Prospects for other hopefuls are by no means certain either. EU treaties must still be amended to allow any new members to join, but scepticism of institutional reform in a number of member countries means that Croatia, at the front of the queue, is likely to have to wait until at least 2010 for entry. Serbia's bid, stalled by its failure to help bring war crimes suspects to trial, was expected to come up at the summit. Leaders will also study how to help states such as Spain, Italy and Malta cope with sudden influxes of migrants and are expected to agree to pool their border guards to prevent illegal migrants reaching the bloc by sea, according to a draft accord.