Stoltenberg was reacting to a declaration from Putin last week that Russia could use nuclear weapons if it was struck with conventional missiles, and that Moscow would consider any assault on it supported by a nuclear power to be a joint attack, Reuters reported.
Putin's warning came as the United States and its allies deliberate over whether to let Ukraine fire conventional Western missiles deep into Russia. Kyiv says it wants permission to hit targets that are part of Russia's war effort.
"What we have seen is a pattern of reckless Russian nuclear rhetoric and messaging, and this fits into that pattern," Stoltenberg, who hands over the NATO leadership to Dutch ex-prime minister Mark Rutte on Tuesday after a decade in charge.
MA/PR