The incident which left ex-spy Sergei Skripal, his daughter and a British police officer in a serious condition has galvanised the West to confront Russia over the incident and what they called a “pattern” of unacceptable behaviour.
The Kremlin is expected to set out retaliatory measures after the UK announced it would expel 23 Russian diplomats earlier this week, freeze assets and seek new powers to deal with “hostile activity”.
Speaking to BBC Radio 4, Mr Stoltenberg said: “I’m absolutely certain that Russia has underestimated the resolve and the unity of Nato allies.”
On Thursday the leaders of France, Germany the US and UK took the unprecedented step of issuing a joint statement blaming Russia for the 4 March Salisbury attack involving a Russian-made Novichok nerve agent.
But they went further saying, the attack was only the latest element of a series of “unacceptable” incidents taking in Russia’s invasion of Crimea and destabilizing eastern Ukraine.
Discussing defences in Eastern Europe, Mr Stoltenberg went on: “We have now reached the level of four battle groups we don’t have any immediate plans to increase the deployment.
“But what we are doing is increasing the ability to reinforce, to send in more troops quickly if needed.
“So we have tripled the size of what we call the Nato Response Force, we also established a high readiness brigade that can be deployed in a matter of days.”
Nato’s four multinational battlegroups, including a British-led one in Estonia, and others in Latvia, Lithuania and Poland, totalling 4,500 troops, became fully operational in 2017.
Three years earlier, the alliance set about increasing the size of the Nato Response Force to 40,000, with the high-readiness Spearhead Force at its core.
Mr Stoltenberg added: “It’s important Russia gets a clears signal that it has consequences, costs, to behave the way they behave.
“All Nato allies express a strong political support for the UK, the United Kingdom is not alone.”
Russia's Ambassador to the UK, Alexander Yakovenko, yesterday described Ms May's expulsion of diplomats as "absolutely unacceptable" and "a provocation" and urged her to "follow international law".
Moscow's envoy to the UN, Vasily Nebenzya, responded to Britain giving his country a 24-hour deadline to answer the poisoning accusations earlier this week by saying Russia "does not speak the language of ultimatums" and protesting that it had been asked to confess without being given the opportunity to carry out its own investigations.
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