The U.S. says Russia isn’t preparing to use nuclear weapons, yet.

Biden administration officials have expressed concern that President Vladimir V. Putin of Russia could detonate a tactical nuclear weapon — perhaps in a demonstration blast over the Black Sea or Arctic Ocean, or in Ukrainian territory — but on Saturday said there is no evidence yet that he is moving those weapons or preparing such a strike.

In a videotaped speech to the Russian people on Wednesday announcing plans to call up an additional 300,000 reservists, Mr. Putin said: “If the territorial integrity of our country is threatened, we will certainly use all the means at our disposal to protect Russia and our people. This is not a bluff.”

At the United Nations General Assembly in New York on Wednesday, President Biden said Mr. Putin was making “irresponsible nuclear threats.”

Some U.S. officials said they noted a change in the tone, scope and seriousness of Mr. Putin’s latest threats, compared with similar ones he made after Russia’s withdrawal from Kyiv and northern Ukraine earlier this year. Two senior officials said they do not believe Mr. Putin is poised to use nuclear weapons now, given the response it would draw from the West as well as from Moscow’s allies like China and India, who last week voiced their sternest criticism to date of the war.

But these U.S. officials also said they are treating Mr. Putin’s remarks very seriously.

President Biden said in an interview with the CBS show “60 Minutes” on Sunday that Mr. Putin would face a “consequential” response from the United States if he used nuclear weapons, but Mr. Biden and other administration officials have declined to specify that response.

Karine Jean-Pierre, the White House press secretary, reiterated that position on Friday in light of Mr. Putin’s latest comments.

“We obviously take these threats very seriously,” she said, “but we have not seen any reason to adjust our own nuclear posture at this time.”