He suggested European nations need to increase the size of their forces, echoing similar calls made by the US and NATO Secretary General Mark Rutte.
Both the US and Rutte want to see the alliance spending considerably more.
Latest figures by the International Institute for Strategic Studies think tank show Russia is now spending more on defence than the whole of Europe combined.
The UK currently spends around 2.3% of its GDP on defence, making it one of the 23 nations out of 32 to be achieving the NATO target of 2%.