"This is a first step, it is encouraging, but we have to realise there is still a lot of hard work that lies ahead of us," Stoltenberg said at a meeting of NATO foreign ministers in Brussels.

North Korean leader Kim Jong Un met the South's President Moon Jae-in and agreed to pursue permanent peace and the complete denuclearisation of their divided peninsula.

Stoltenberg said the landmark meeting had come about as a result of the intense political, diplomatic and economic pressure the international community had exerted on Pyongyang.

"The most important thing today is to welcome the fact they have met, and even though there's a long way to go before we see a full resolution to the crisis and the problems we see on the Korean peninsula, I think this is a very important first step," the former Norwegian premier said.