The ministers will open their session in Brussels in the afternoon with a debate on the goals and the agenda of the summit in Celtic Manor near Cardiff on 4 and 5 September. The summit will focus on the alliance's future after 2014, also with respect to its role in Afghanistan.

A key priority is boosting the cross-Atlantic ties, so that these are "strong and balanced", also from the aspect of the allies' financial contributions, sources in NATO say. It is not clear yet, however, whether the summit will confirm any concrete long-term goals concerning defence budgets.

A vital aspect of discussion about NATO's future is the question whether Afghanistan will invite NATO to continue its presence in the country after 2014. NATO is due to end its combat mission there at the end of 2014, while it would continue to provide training and advice in the follow-on Resolute Support mission.

The ministers will also discuss the alliance's open door policy and the Western Balkans. They are expected to confirm drawing up a progress report on four candidate countries - Montenegro, Macedonia, Bosnia-Herzegovina and Georgia.

The report is to determine what the aspiring countries can expect from the next year's summit. Based on that report, the allies would then discuss the enlargement aspect of the summit broader in June.

Slovenia has been warning of the problem of the EU's enlargement fatigue moving to NATO, underscoring that NATO rather than EU criteria should be taken into consideration in the implementation of NATO's open door policy.

NATO sources also say that enlargement to post-Yugoslav area should not be a victim of the failed attempt at enlargement to the post-Soviet region as seen in Bucharest.

Slovenia is strongly in favour of NATO confirming its open door policy in September by extending an invitation to one of the candidate countries. Slovenia believes the country best prepared to make the step is Montenegro.

As the summit is nearing, debate is also expected on potential candidates to succeed Anders Fogh Rasmussen in the post of NATO secretary general. Slovenia supports Italian Franco Frattini.

Erjavec will attend NATO ministerial only on Tuesday as he will join PM Alenka Bratušek on an official visit to Paris on Wednesday.