A Russian nuclear strike would change the course of the conflict and almost certainly provoke a “physical response” from Ukraine’s allies and potentially from the North Atlantic Treaty Organization, a senior NATO official said on Wednesday.

Any use of nuclear weapons by Moscow would have “unprecedented consequences” for Russia, the official said on the eve of a closed-door meeting of NATO’s nuclear planning group on Thursday.

Speaking on condition of anonymity, he said a nuclear strike by Moscow would “almost certainly be drawing a physical response from many allies, and potentially from NATO itself”.

Still Moscow appears to be using its nuclear threats mainly to deter NATO and other countries from directly entering its war on Ukraine, he added.

More than seven months into the invasion of Ukraine, Russia has suffered huge losses in men and equipment and been beaten back on several fronts within the last month.

President Vladimir Putin in response has proclaimed the annexation of Ukrainian territories only partly under Russian control, and threatened to defend them with nuclear weapons.

U.S. National Security Adviser Jake Sullivan said last month the United States has made clear to Moscow the “catastrophic consequences” it would face if it used a nuclear weapon in Ukraine.

Sullivan did not publicly describe the nature of the planned U.S. response, however.

U.S. Defense Secretary Lloyd Austin, speaking to reporters at NATO headquarters on Wednesday, said the United States was monitoring any indications of a worrying change in Russia’s nuclear posture that would suggest Putin might move ahead with a nuclear strike.

“We’ve not seen any indicators at this point that would lead us to believe that,” Austin said.

Diplomats say Moscow is trying to scare the West into reducing its support for Kyiv by hinting at the use of a tactical nuclear weapon to defend the annexed territories of Ukraine.

Meanwhile, NATO said it would go ahead with its annual nuclear preparedness exercise dubbed “Steadfast Noon” next week, in which NATO air forces practise the use of U.S. nuclear bombs based in Europe with training flights, without live weapons.

Cancelling the drills because of the war in Ukraine would send a “very wrong signal,” NATO chief Jens Stoltenberg said on Tuesday.

“It’s an exercise to ensure that our nuclear deterrent remains safe, secure and effective,” he said, adding NATO’s military strength was the best way to prevent any escalation of tensions.