U.S. warns Moscow against using chemical or biological weapons.

President Biden’s national security adviser, Jake Sullivan, warned his Russian counterpart on Wednesday against “any possible Russian decision to use chemical or biological weapons in Ukraine,” the White House said in a statement.

The explicit warning to Nikolai P. Patrushev, President Vladimir V. Putin’s main national security adviser, reflected escalating concerns in Washington that the Russians, stymied in their hopes of a quick takeover of the country, could resort to weapons of mass destruction.

Officials said there was no direct mention of the use of battlefield nuclear weapons, although two officials said the administration sent a separate warning on that issue through other channels in the opening days of the war, when Mr. Putin announced he was placing Russian nuclear forces on alert.

The White House would not say who initiated Wednesday’s call, though Russia indicated it came at the request of the United States. Read more