BELGRADE, Sept 30 (Reuters) - The European Commission moved on Thursday to reassure Serbia of its future European Union membership after an internal document in Brussels showed that EU states could no longer agree to guarantee six Balkan countries a place in the bloc. Separately, France said it supported opening formal EU membership talks with Albania and North Macedonia, which hope to join Serbia and Montenegro as official candidates actively preparing to join the bloc, provided they pursue reforms. Ahead of a Balkan-EU summit on Oct. 6 in Slovenia, Reuters reported on Tuesday that the 27 member states have so far been unable to agree a declaration reaffirming their 18-year-old pledge of future EU membership for the western Balkan states. read more European Commission chief Ursula von der Leyen used her stopover in Serbia on Thursday, part of a six-nation regional tour, to show that Brussels has not forgotten about their aspirations. "I am a strong advocate for bringing Serbia into the European Union," she said in a speech to open a railway. "We support Serbia"s ambition to open as soon as possible new accession clusters," von der Leyen said, referring to negotiating chapters, while also acknowledging that EU states had the final word in allowing Belgrade to move forward. Serbia, the largest non-EU Balkan country with about seven million people, is seen as the linchpin in the region and the EU hopes Belgrade"s influence in the Balkans could help others to reform. The remaining two aspiring EU members are Bosnia-Herzegovina and Kosovo. BLOCKAGE Bulgaria has blocked North Macedonia from starting its membership talks over a cultural and language dispute. Albania is tied to North Macedonia in the accession process under EU rules. French foreign ministry spokesperson Anne Claire Legendre told Reuters that, given reforms to the EU"s accession methodology, the conditions were right for an "intergovernmental conference" to launch membership talks with Skopje and Tirana. In February 2018, the European Commission said Serbia could join the EU by 2025, though it added this was a very ambitious goal. That now looks out of reach as slow progress in rule-of-law reforms are holding up accession negotiations. Serbia"s President Aleksandar Vucic, standing alongside von der Leyen, promised reform and to help improve ties with Kosovo, which declared independence from Serbia in 2008. Vucic, a former ultra-nationalist who pivoted towards a pro-European stance in 2008, aims to bring his country into the EU, which requires some form of accommodation with Kosovo. But Serbians see Kosovo as an inseparable cradle of their national identity and do not recognise its independence. In a positive step, Kosovo agreed on Thursday to withdraw police units from its northern border with Serbia to end an escalation over free movement between the two countries that prompted NATO to step up its patrols. read more