Mr Stoltenberg said Russian President Vladimir Putin’s behaviour and attempt to establish a new “sphere of influence” would see a prompt response from NATO. He added: “We don’t want a new Cold War. We don’t want a new arms race.
“But, at the same time we have to make sure that we are adapting as the world is changing.
“So we are responding to what Russia is doing.”
Mr Stoltenberg warned Russia’s aggressive actions have been noted against its neighbours in eastern Europe.
He said the West is witnessing a Russia which is trying to reestablish some kind of sphere of influence similar to the Soviet Union during the Cold War.
He added: “We have seen that in Georgia, in Moldova, in Ukraine, and that requires a response from NATO.
“And that’s exactly why we now are implementing the biggest reinforcement of our collective defence since the end of the Cold War.
The NATO chief described how the nations that comprise the world’s largest military organisation was not cutting defence spending.
He said: “All Nato allies are now increasing defence spending and we are modernising our armed forces.
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The poisoning of Mr Nalvany follows the assassination of Alexander Litvinenko in London and the attack on Sergei Skripal and his daughter Yulia in Salisbury.
Speaking to the Daily Telegraph Mr Stoltenberg said: “What matters now is that Russia has to answer some very serious questions”.
He added: “How can this happen and how can we see again and again that opposition leaders in Russia are attacked and their lives threatened and some actually killed?
“This is not the first time we’ve seen the use of poison against people who are in opposition to the Russian regime. That makes it even more serious.”
The NATO chief said he wished Mr Nalvany “a speedy recovery”.
He added: “It is an attack on fundamental democratic rights.
“An attack on the right to be in opposition, the right to be an outspoken critic of the regime.”
Mr Stoltenberg is now demanding Russia give “complete disclosure” of its Novichok programme.
In a message to President Putin, he argued Russia must transparently provide information on Novichok to the international Organisation for the Prohibition of Chemical Weapons.
He stressed those responsible for the assassination attempt “have to be held accountable”.
He added: “And that’s exactly what we are clearly conveying to Russian authorities.
“NATO countries will coordinate the way forward, but it’s a bit early to say anything more.”