The meeting will be hosted by the presidents of the two countries behind the initiative, Slovenia's Borut Pahor and Croatia's Kolinda Grabar-Kitarović.

The extraordinary summit will be the first time that the leaders of the informal regional club, established in 2010 to promote integration and efforts to join the EU, dedicate their talks to security issues. The debate has been prompted by the refugee crisis, which is stoking tensions among the countries on the Balkan migration route.

Managing the massive flow of refugees has proved a major political challenge in the countries on the route and the inability to agree on a common European policy has further hindered the scramble for the right response. Raising the stakes in recent days is the fear that the masses of refugees could be infiltrated by Islamic extremists.

The meeting will therefore look to provide a new impetus in efforts to deal with the crisis and reaffirm the commitment of all countries to EU integration, a key driving force of reforms in the region.

Heads of state of all participating countries in the Brdo-Brijuni Process - Slovenia, Croatia, Serbia, Bosnia-Herzegovina, Montenegro, Macedonia, Kosovo and Albania - have confirmed their attendance.

The significance of the meeting has not been lost on international partners and the EU and US will both be represented at a high level, with European Council President Donald Tusk and US Vice-President Joe Biden. Austrian President Heinz Fischer and German Foreign Minister Frank-Walter Steinmeier are also expected to attend.

The primary aim of the summit is to reduce security risks in the region by building trust. To help the effort, the countries in the region need reassurances of clear prospects for EU membership, President Pahor has said.

"The summit comes at the right time, as the perception by people of a threat has increased due to recent events. The politicians now have a responsibility to take measures...for securing peace and security," Pahor said in an interview for the STA ahead of the event.

The Brdo-Brijuni Process has been a platform for aspiring EU members to obtain important experience from Slovenia and Croatia and a channel to remind the bloc of the need to commit to enlargement. The Zagreb meeting should stress the need for the EU to retain its stabilising role through an open-door policy.

"The core message is that the prospects of EU membership of this region must be reaffirmed. Isolation of these countries outside of European integration processes would only increase instability," said Pahor.

Another message that the meeting will look to provide is that each country in the region has the right to freely choose the international organisations of which they want to be a part, according to diplomats laying the groundwork for the summit.

While all who are not yet members aspire to join the EU, there are more pronounced differences when it comes to their desire to join security organisations.

Serbia in particular has said it is not interested in joining NATO and the freedom of choice is aimed at preventing the region from becoming a playground of geostrategic interests. Additionally, it has allowed the countries in the club to freely debate security issues, which they have avoided in the past.

While the presidents largely hold ceremonial roles in their countries, Pahor has rejected the notion that these were "ceremonial conferences". "The conferences of the Brdo-Brijuni Process have moved the limits of what is possible in terms of cooperation, the strengthening of peace and the promotion of reconciliation."

"What is even more important is that they have helped to influence public opinion in the member states and opinion in the international community. In the participating countries, the message has foremost been one of hope that cooperation and the overcoming of differences is possible."

Efforts to achieve regional cooperation in SE Europe often require the surpassing of bilateral differences between individual members and the meeting in Zagreb should again be an opportunity for this.

Previous summits have resulted in a thawing of relations between Serbia and Kosovo that led to a historic meeting in 2013 between the presidents of the two countries.

With the focus on the refugee crisis, it is expected that Serbian president Tomislav Nikolić and Grabar-Kitarović will meet on the sidelines, after their countries became engaged in a spiteful tit-for-tat over the handling of the issue in October.

For Slovenia the priority in the refugee crisis is limiting the flow on the Balkans route so the country can re-establish normal functioning of the Schengen system of border control. While it has recently started to erect a razor wire fence along the border with Croatia to aid these efforts, Pahor said an agreement with the southern neighbour should be the main priority.

Perhaps most importantly, a successful summit in Zagreb could lay the foundations for efforts to reach out to countries lower on the Balkan route in a bid to manage the refugee flow as part of a comprehensive solution.