Nato Secretary General Jens Stoltenberg said on Thursday that Russian President Vladimir Putin would be crossing a “very important line” if he were to order the use of nuclear weapons in Ukraine, as both the military alliance and Russia are due to hold nuclear exercises in the next few days.
Mr Stoltenberg's comments come just before Nato holds its exercise, named “Steadfast Noon,” next week.
The long-planned manoeuvres are conducted about the same time every year and run for about a week.
They involve fighter jets capable of carrying nuclear warheads, but do not involve any live bombs.
Nato is expecting Moscow’s exercise of its nuclear forces some time this month as Russia usually holds its own manoeuvres about the same time.
Mr Stoltenberg said Nato would “closely monitor” what Russia was up to.
Asked what Nato would do if Russia launched a nuclear attack, he said: “We will not go into exactly how 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, fundamentally changing the nature of the war in Ukraine, and of course that would have consequences.”
Mr Stoltenberg was speaking after a meeting of Nato’s secretive Nuclear Planning Group, which was held among defence ministers in Brussels, as concerns deepen over Mr Putin’s insistence that he would use any means necessary to defend Russian territory.
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The meeting, which usually happens once or twice a year, comes amid high tension as some Nato allies supply Ukraine with advanced weapons and munitions to defend itself against Russian aerial attacks.
“Irresponsible and reckless rhetoric is dangerous,” US Defence Secretary Lloyd Austin said of Russian threats to potentially use nuclear weapons in Ukraine.
But the US has not seen the need to make any changes to its current military posture or response, Mr Austin said.
“Any nuclear attack against Ukraine will create an answer, not a nuclear answer but such a powerful answer from the military side that the Russian Army will be annihilated,” he said in a speech in Bruges, Belgium.
Nato is keeping a wary eye on Russia’s movements in its war with Ukraine, but has so far seen no change in its nuclear stance.
Mr Putin’s nuclear exercises, though, could make it more difficult for Nato to understand what Russia’s intentions might be, possibly increasing the risk of an accident.
“Russia will also be conducting its annual exercise, I think, the week after or just after the annual exercise,” UK Defence Secretary Ben Wallace said on Wednesday. But “what we don’t want is to do things out of routine".
“This is a routine exercise and it’s all about readiness,” Mr Wallace said, just as “Nato’s meeting is all about making sure we are ready for anything.
"I mean, that is the job of this alliance, to make sure that the 30 partners together are ready for what is thrown at us. And we have to continue to work at that.”
Vladimir Putin says Russia will use 'all means' to defend its territory - video
Fourteen Nato member countries will be involved in “Steadfast Noon,” which was planned before Russia invaded Ukraine on February 24.
The main part of the manoeuvres will be held more than 1,000 kilometres from Russia.
But France insists on maintaining its nuclear independence and does not take part in Nuclear Planning Group meetings.
With the Russian army retreating in some places when faced with Ukrainian forces armed with western weapons, Mr Putin raised the stakes by annexing four Ukrainian regions and declaring a partial mobilisation of reservists to strengthen the failing front line.
As his war plans have gone awry, Mr Putin has repeatedly indicated that he could resort to nuclear weapons to protect Russian gains.
The threat is also aimed at deterring Nato nations from sending more sophisticated weapons to Ukraine.
French President Emmanuel Macron suggested to France 2 TV that the country would not respond with a nuclear strike.
He also warned about the responsibilities of leaders when it comes to nuclear talk and said he has spoken to Putin “several times.”
“We have a [nuclear] doctrine, which is clear,” Mr Macron said.
“The dissuasion is working. But then, the less we talk about it, the less we brandish the threat, the more credible we are.
“Too many people are talking about it."