Vladimir Putin made 'big mistake' invading Ukraine: NATO chief Jens Stoltenberg

Russian President Vladimir Putin

NATO chief Jens Stoltenberg on Thursday accused Russian President Vladimir Putin of making a "big mistake" by invading Ukraine, as leaders gathered to discuss overhauling the alliance's eastern defences.

"President Putin has made a big mistake and that is to launch a war against an independent sovereign nation. He has underestimated the strength of the Ukrainian people, the bravery of the Ukrainian people and their armed forces," Stoltenberg said ahead of the start of a summit in Brussels.

Stoltenberg said the leaders of the US-led military alliance would "address the need for a reset of our deterrence and defence in the longer term", starting with agreeing new deployments to eastern members Romania, Hungary, Slovakia and Bulgaria.