He made the comment in an excerpt of an interview scheduled to be broadcast on LBC Radio’s Nick Ferrari at Breakfast programme on Friday.
“It’s very, very important that we shouldn’t allow ourselves to be sidetracked by this kind of sabre-rattling, because fundamentally, what Putin is trying to do is to reframe this [as] ‘it’s about Russia versus Nato’. It’s about, you know, a stand-off of that kind,” Mr Johnson says.
“It’s not. It’s about his attack on an entirely innocent country, with conventional weapons, with artillery, bombardments with planes, shells and so on and it’s about the Ukrainians’ right to protect themselves.
“That is what this is about and what we had, at Nato, was yet again, the alliance being tested, being asked, being interrogated. Are we resolved? Are we determined? Will we give the Ukrainians the means to protect themselves?
“And the answer was absolutely yes and, if anything, the strength of the unity is greater than it was before.”
Mr Johnson’s scheduled appearance comes after an opinion piece written by the former British ambassador to Washington, Kim Darroch, in The Daily Telegraph in which she warns Nato that Mr Putin may “resort to nuclear weapons” over the war in Ukraine.
The prime minister described the war in Ukraine as a “wanton, illegal, unprovoked active aggression against a sovereign independent country”.
Asked whether Nato had acted as a united alliance during its Madrid summit this week, Mr Johnson said the conversations are now over “changing the dynamic” of the conflict.
Commenting further on the withdrawal of Russian forces from a strategic Black Sea island on Thursday, Mr Johnson said the pull-out from Snake Island was a “lesson”.
The Kremlin portrayed the pull-out as a “goodwill gesture” but Ukraine’s military claimed it had forced the Russians to flee in two small speedboats following an artillery barrage and missile strikes.
Nato leaders on Thursday vowed to support Ukraine for as long as it takes to avoid defeat at the hands of Russia as President Joe Biden declared the US would send more military aid to Kyiv, including a “new advanced air defence system”.
Mr Biden said another $800 million in defence assistance for Ukraine would be announced in the next few days.
The UK will also ship another $1.2 billion in weapons to Ukraine, comprising 5,000 shoulder-fired missiles and other weaponry.