In a speech on Tuesday, he also said the alliance is monitoring Russia’s nuclear forces closely as the country was “losing on the battlefield” in Ukraine.
Meanwhile, Moscow has issued a fresh warning to the West over its involvement in the Ukraine war.
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Deputy foreign minister Sergei Ryabkov said Russia will take adequate countermeasures in response to the West’s “growing involvement”.
In the comments reported by the state-owned RIA news agency, he said: “We warn and hope that they realise the danger of uncontrolled escalation in Washington and other Western capitals.”
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Russian bombs have rained down on Ukraine, killing at least 14 people on Monday.
Despite remarkable Ukrainian battlefield successes – both early on in the war with the defence of Kyiv and more recently with counterattacks in Kharkiv and Kherson regions – the war could continue for decades to come, one expert said.
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September: NATO ‘could sink whole Black Sea Fleet’
Sky News security and defence analyst Professor Michael Clarke said the war is likely to be a “generational struggle” and could be a “forever conflict” until “something changes in European security or Russia”.
Mr Clarke said the current crisis in Ukraine was the “second war” and the first war was witnessed in 2014 when Russia annexed Crimea.
He added: “My feeling is next year there will be a ceasefire in which the Ukrainians will be better placed and that ceasefire will be unstable and it will break down and there will be a third war and then a ceasefire and a fourth war.
“We’re dealing here with an existential struggle because the Russian establishment thinks that Ukraine has no right to exist and they won’t change their mind in the short-term.
“This is likely to be a generational struggle. Let’s say it’s going to last 30,40 or 50 years.”