SARAJEVO (Reuters) - The European Union told Balkan leaders on Thursday to quit “playing with fire” and set aside their political quarrels in order to take advantage of what was described as the bloc’s renewed concern for the region.

EU commissioner Johannes Hahn was speaking at a summit of six Balkan countries that want to join the EU but are growing concerned at the resistance from some EU states to taking in any new members from ex-communist Eastern Europe.

The prospect of EU membership has for years been the main driver of reform in the Balkans after a decade of war and upheaval in the 1990s.

But with Britain’s vote to leave the bloc, the rise of right-wing populism and debate over the future shape of the EU, expansion has been placed firmly on the backburner.

Analysts and diplomats say this has severely diminished the EU’s leverage in the Balkans, an overwhelmingly poor region troubled by ethnic tensions, endemic corruption and a worrying drift towards authoritarianism. The West says Russia is taking advantage, forging alliances and exacerbating tensions.

After an EU summit last week at which the bloc reiterated its commitment to the region’s “European perspective”, Hahn met the prime ministers of Albania, Bosnia, Macedonia, Montenegro, Kosovo and Serbia in the Bosnian capital Sarajevo.

“I really cannot recall a time like now when member states and others actively approached – even pushed - me to check what items could be quickly delivered to the Western Balkans to support the region,” he told the summit in a televised session.

“Use this rare window of opportunity,” he said. “I don’t think you can afford to squander this positive climate through domestic confrontations and blaming neighbors.”

Otherwise, he said, “we end up in a really awkward spot - with a stream of bad news slamming the window firmly shut. This is playing with fire.”

Hahn, who deals with the EU’s expansion to other countries, appeared to allude to Russia when he warned of “unprecedented levels of involvement from further east” and said United States policy towards the Balkans under President Donald Trump was still unclear.

The summit, organized by Bosnia and the EU, focussed on regional economic cooperation and infrastructure projects.

It was held against a backdrop of political crisis in Macedonia, parliamentary boycott in Montenegro, angry exchanges between Serbia and its former Kosovo province and the threat of further fragmentation in Bosnia.

Each of the six countries represented are at various stages on the road to EU accession. They discussed the possibility of forming a regional common market of 20 million people.

But Serbian Prime Minister Aleksandar Vucic was skeptical.

“It’s nice that we shall produce yet another declaration,” he told the Serbian state news agency Tanjug. “Only, as soon as this day is over, we’ll have fights and clashes again.”