The director, William J. Burns, served as American ambassador to Russia and is the member of the administration who has dealt most often with Mr. Putin. He said the potential detonation of a limited nuclear weapon — even as a warning shot — was a possibility that the United States remained “very concerned” about. But Mr. Burns quickly cautioned that so far, despite Mr. Putin’s frequent invocation of nuclear threats, he had seen no “practical evidence” of the kinds of military deployments or movement of weapons that would suggest such a development was imminent.

“Given the potential desperation of President Putin and the Russian leadership, given the setbacks that they’ve faced so far, militarily, none of us can take lightly the threat posed by a potential resort to tactical nuclear weapons or low-yield nuclear weapons,” Mr. Burns said. “We don’t.”