"NATO remains ready and able to defend all Allies," he declared in a statement on Sunday.
"Any attack on NATO will be met with a united and forceful response," he added.
Stoltenberg was reacting to remarks by former US President Donald Trump, who stated a day earlier that Russia should be able to do “whatever the hell they want” to NATO countries that do not meet their defence spending targets.
Polish Prime Minister Donald Tusk said on Sunday: “We have a hot war at our border,” referring to Russia's invasion of neighbouring Ukraine.
He voiced concerns about whether the United States would show “full solidarity with other NATO countries in this confrontation that promises to last for a long time with Russia.”
Polish Defence Minister Władysław Kosiniak-Kamysz said on the X social media platform: "NATO's motto 'one for all, all for one' is a concrete commitment."
He added: "Undermining the credibility of allied countries means weakening the entire NATO. No election campaign is an excuse for playing with the security of the Alliance."
Russia invaded Ukraine on February 24, 2022, starting the largest armed conflict in Europe since World War II.
Monday is day 719 of Russia’s war on Ukraine.
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Source: Reuters, AP, IAR, PAP