The United Nations said it was dispatching a team to assist in the investigation, while the Biden administration warned the Kremlin against using unconventional weapons to reverse the successful counteroffensive by Ukrainian troops.

In an interview with the CBS News program “60 Minutes” that is scheduled to air on Sunday, President Biden said that the United States’ response to any use of such weapons would be “consequential,” though he did not provide details. …

With Russia suffering military setbacks in the past weeks, and the Kremlin growing increasingly isolated diplomatically, the Biden administration warned Mr. Putin against using unconventional weapons, marking the second time that Washington has issued such a warning. Last March, Mr. Biden said that “we would respond” if Mr. Putin — then frustrated by setbacks in Kyiv, the Ukrainian capital, and by the general lack of progress in the war — were to use such weapons.

In the “60 Minutes” interview, Mr. Biden said that Russia’s use of unconventional or nuclear weapons to try to turn the tide of the war in Moscow’s favor would “change the face of war unlike anything since World War II.”