He described the current climate as the most difficult he had experienced in his 30 year career.
“It is more unpredictable, and it’s more difficult because we have so many challenges at the same time,” Mr Stoltenberg told the Guardian.
“We have proliferation of weapons of mass destruction in North Korea, we have terrorists, instability, and we have a more assertive Russia. It is a more dangerous world.”
Mr Stoltenberg made the comments while in Estonia where he was visiting British troops.
The visit comes ahead of a military exercise to be held by almost 100,000 Russian and Belarusian troops. It is expected to be the largest of its kind since the Cold War, although Russia has said the exercises will only involve around 13,000 troops.
“Russia has said it is below 13,000. They briefed that on the Nato-Russia council a few weeks ago,” Mr Stoltenberg said. “That was useful but at the same time we have seen when Russia says that an exercise has less than 13,000 troops that’s not always the case. We have seen that in Zapad 2009 and 2013 – the two previous Zapad exercises. There were many more troops participating.”
On the other side of the world, North Korea has continued to ramp up its nuclear programme and threaten the US, leading to Donald Trump saying he would unleash “fire and fury” should it continue.
South Korea, fearful of an attack from its neighbour, has deployed a controversial US missile defence system.
Hundreds of protestors surrounded the military base as the defence system arrived, amid fears it could lead to environmental and health problems.
After witnessing the North launch a ballistic missile over Japan, the South Korean prime minister, Lee Nak-yeon, said “a special measure is urgently needed to stop their recklessness”.
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