Participants, among them also the Slovenian foreign and defence ministers, Karl Erjavec and Andreja Katič, are to focus on framing a readiness action plan to deter the increasingly aggressive Russia, which intervened in eastern Ukraine and annexed Crimea in 2014.

The crisis in Ukraine was a serious warning to NATO, which is why the alliance wants to use the Warsaw summit as an opportunity to send a clear message to Russia that it would not fall asleep again. In response to the crisis in Ukraine, NATO tripled the number of soldiers in the NATO Response Force to 40,000.

Poland, Estonia, Latvia and Lithuania have meanwhile requested a stronger presence of the alliance's forces on their territory, which they believe will deter Russia from aggression.

Top EU and NATO representatives will thus sign a joint declaration highlighting the importance of further strengthening EU-NATO cooperation at a time of major security challenges, including hybrid threats and cyber security, in NATO's southern and eastern neighbourhood.

Brexit will also be high on the agenda because of fears that Russia could exploit the UK's negotiations over leaving the EU in order to sow disunity amongst member states and weaken their resolve.

NATO leaders will confirm plans to encourage stability by providing help to southern Europe, which is facing threats due to illegal immigrations across the Mediterranean and terrorist groups, such as Islamic State. NATO said it was aware of its role in the issue, but believes the EU should lead the efforts.

The Warsaw summit will also go over the implementation of commitments made at the 2014 summit in Wales, including increasing NATO members' defence spending to 2% of GDP after the spending was being cut during the financial crisis.

Slovenia is near the bottom in defence spending, allocating only 0.94% of GDP for defence in 2015. The same share is planned for this year, with most of the money going for salaries, while equipment procurement would get only 1.17% when it should be getting 20% in line with NATO's goals.

According to the prime minister's office, Cerar will give an address at the summit to say that Slovenia had stopped the falling trend in defence expenditure and that it aimed to increase the percentage of GDP for defence from 2018 on.

The opposition United Left (ZL) already announced it would oppose this, arguing that the move would improve neither the financial situation of soldiers nor the country's defence capabilities. The party also expressed opposition to other expected conclusions of the summit.

The Warsaw summit is expected to be attended 18 presidents, 21 heads of government, 41 foreign ministers and 39 defence ministers. More than 2,000 journalists from around the world are also expected to report on the event.