The group is facing one its most difficult meetings in years due to US President Donald Trump's criticism of the organisation and his targeting of German leader Angela Merkel.

But with the World Cup semi-finals in NATO's traditional adversary Russia made up exclusively of members of the western military alliance, the heads of state and government in Brussels were able to turn to football to lighten the atmosphere.

British Prime Minister Theresa May exchanged a team jersey with her Croatian counterpart ahead of Wednesday's night's England-Croatia semi final clash.

Luxembourg Prime Minister Xavier Bettel tweeted a pictured of May smiling with Croatian President Kolinda Grabar-Kitarovic, saying that everything was "still all ok" between the on-pitch rivals.

The Croatian president also gave May a shirt -- and one to Trump with his surname emblazoned on the back, even though the United States did not compete in the World Cup finals.

Belgian Prime Minister Charles Michel meanwhile teased French President Emmanuel Macron about Tuesday's game between their countries that saw France win 1-0 to book a place in the final on Sunday.

"Frankly Emmanuel, yesterday the French team was very lucky," Michel said with a smile.

But he suggested the sport could provide an example to the military alliance.

"Like in football, fair-play and team spirit are what we need more than ever," he said.

NATO chief Jens Stoltenberg declined to say which team he was supporting when asked by reporters.

"Representing 29 nations I will not pick any specific team I would like to win," he said, adding that his own country Norway had not qualified for the finals in Russia.