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Secretary General Jens Stoltenberg said the alliance was deploying — for the first time as a defensive unit — parts of its combat-ready response force and would continue to send defence support to Ukraine.
Mr Stoltenberg did not say how many troops would be deployed, but confirmed that the move would involve land, sea and air power.
“We are deploying the Nato response force for the first time in a collective defence context. We speak about thousands of troops. We speak about air and maritime capabilities,” he said.
“Several allies have already assigned troops and forces to the Nato response force.
“We have the high readiness force, the Nato response force, and that is why we are deploying it. To prevent any misunderstanding that we are not prepared to protect all allies.”
The force can number up to 40,000 troops, but Mr Stoltenberg would only say that the entire force would not be deployed immediately.
Nato previously had about 5,000 troops stationed in Poland and the Baltic countries Estonia, Latvia and Lithuania.
“Russia has shattered peace on the European continent. What we have warned against for months has come to pass despite all of our efforts to find a diplomatic solution,” Mr Stoltenberg said.
The UK government said it is prepared to meet any further request from Nato’s Supreme Allied Commander Europe to extend military support.
UK Prime Minister Boris Johnson warned the alliance that Russian President Vladimir Putin’s ambitions might extend beyond Ukraine.
A Downing Street spokeswoman said: “The prime minister told the group that a catastrophe was engulfing Ukraine and President Putin was engaging in a revanchist mission to overturn post-Cold War order.
“He warned the group that the Russian president’s ambitions might not stop there and that this was a Euro-Atlantic crisis with global consequences.”