Nato will test its ability to launch a nuclear strike in a training exercise over the UK, North Sea and Belgium next week, the alliance said on Friday.
The decision to flag the Steadfast Noon drills is an unusual step and a sign of global tensions around nuclear weapons after Russian President Vladimir Putin stoked fears he would use them in Ukraine.
The exercise will see up to 60 aircraft from air forces belonging to 14 countries take part. While no live weapons will be used, planes with the capability of carrying B61 bombs and US B52 bombers will be involved.
"This exercise helps ensure that the alliance's nuclear deterrent remains safe, secure and effective," said alliance spokesperson Oana Lungesco.
Under more normal circumstances, Steadfast Noon would be surrounded by a wall of silence, with comments on anything nuclear-related deemed provocative.
The secrecy around nuclear weapons is also designed not to give any clues as to when the alliance would consider their deployment. Previously, Nato said the circumstances would be "extremely remote".
In the wake of Russia's nuclear threats, Nato Secretary General Jens Stoltenberg earlier this week said it would send the "wrong signal" to cancel the exercise.
The view was endorsed by UK Defence Secretary Ben Wallace on Thursday.
"I think that is the key. What we don't want is to do things out of routine," he said on Thursday.
Of Nato's 30-strong membership, only the US, UK and France possess nuclear weapons, although France isn't part of the alliance's nuclear planning group.
French President Emmanuel Macron put further 'nuclear' water between his country and the US and UK when he said France would not respond with nuclear weapons should Russia use them against Ukraine.
“Our doctrine rests on the fundamental interests of the nation," Mr Macron told public broadcaster France 2 on Wednesday.
"They are defined clearly and wouldn’t be directly affected at all if, for example, there was a ballistic nuclear attack in Ukraine, in the region."
It was the first time Mr Macron has discussed France’s nuclear deterrence doctrine regarding Ukraine in detail, but he said it was not good to talk about it too much.
