Any use of nuclear weapons by Russia in Ukraine will have serious consequences but NATO will not spell out exactly how it would respond, the alliance's Secretary-General Jens Stoltenberg said on Thursday.
"It will have severe consequences if Russia uses any kind of nuclear weapon against Ukraine," Stoltenberg told reporters.
"We will not go into how exactly we will respond but of course this will fundamentally change the nature of the conflict. It will mean that a very important line has been crossed. Even any use of a smaller nuclear weapon will be a very serious thing, changing the nature of the war in Ukraine."
Stoltenberg said NATO would remain "very vigilant" in the coming weeks as Russia holds its own nuclear drills.
