Security officials and diplomats said the intelligence, which they declined to detail, pointed to possible preparations by Russia for deploying chemical munitions, and warned the Kremlin may seek to carry out a “false-flag” attack that attempts to pin the blame on Ukrainians, or perhaps Western governments. The officials, like others quoted in this story, spoke on the condition of anonymity because of the sensitive nature of the matter.

The accusations surfaced as Russia repeated claims that the United States and Ukraine were operating secret biological weapons labs in Eastern Europe — an allegation that the Biden administration dismissed as “total nonsense” and “outright lies.”

Any use of poison gases in Ukraine would violate a decades-old international treaty banning such weapons, and represent a dangerous turn in Russia’s two-week-old military offensive against its neighbor. Russia, which possessed vast stocks of chemical and biological weapons during the Cold War, has used outlawed nerve agents in at least two assassination attempts against political foes of President Vladimir Putin in the past three years, including at least once outside its borders, Western intelligence agencies concluded.