Russia’s announcement on Friday that it had expelled six British diplomats was a familiar chess move in the diplomatic gamesmanship between rivals. What stood out as more ominous was President Vladimir V. Putin’s warning the night before: that a decision to let Ukraine use Western weapons to fire deeper into Russia meant that NATO was “at war” with Russia.

Mr. Putin has made other bellicose threats in response to the West’s stepping up military aid to Ukraine. He has never, though, followed through with any kind of conventional military attack against NATO. It is hard to know whether this time will be different.

Still, Thursday’s warning was one of his most direct statements yet about the prospect of war between NATO and Russia. And he went out of his way to make it, taking the time to record a statement to a state television reporter while at an event St. Petersburg.

If the decision is made to allow Ukraine to fire Western weapons deeper into Russia, Mr. Putin said, “it will mean nothing short of direct involvement — it will mean that NATO countries, the United States, and European countries are parties to the war in Ukraine.”

“This will mean that NATO countries — the United States and European countries — are at war with Russia,” he said, “And if this is the case, then, bearing in mind the change in the essence of the conflict, we will make appropriate decisions in response to the threats that will be posed to us.”

On Friday, Mr. Putin’s spokesman followed up to emphasize the Russian leader’s words were “very important” and “extremely clear.” And Vasily Nebenzya, Russia’s permanent representative to the United Nations, doubled down on Mr. Putin’s rhetoric.