"We don't see, as an intelligence community, practical evidence at this point of Russian planning for the deployment or even potential use of tactical nuclear weapons," Burns said.

"Given the kind of saber rattling that ... we've heard from the Russian leadership, we can't take lightly those possibilities," Burns told a conference hosted by the Financial Times.

"So we stay very sharply focused as an intelligence service ... on those impossibilities at a moment when the stakes are very high for Russia," he said.

Russia placed nuclear forces on high alert shortly after Russian President Vladimir Putin sent troops to Ukraine on February 24. Putin has also made thinly-veiled threats hinting at willingness to deploy Russia's tactical nuclear weapons.

The Kremlin chief has warned of a "lightning fast" retaliation if the West directly intervenes in the Ukraine conflict.

Observers say that in recent days, Russia's state television has attempted to make nuclear weapons use more palatable to the public.