President Joe Biden said on Monday the threat of Russian President Vladimir Putin using tactical nuclear weapons is "real", days after denouncing Russia's deployment of such weapons in Belarus.
On Saturday, Biden called Putin's announcement that Russia had deployed its first tactical nuclear weapons to Belarus "absolutely irresponsible."
"When I was out here about two years ago saying I worried about the Colorado river drying up, everybody looked at me like I was crazy," Biden told a group of donors in California on Monday.
"They looked at me like when I said I worry about Putin using tactical nuclear weapons. It's real," Biden said.
Last week, Belarusian President Alexander Lukashenko said his country has started taking delivery of Russian tactical nuclear weapons, some of which he said were three times more powerful than the atomic bombs the U.S. dropped on Hiroshima and Nagasaki in 1945.
The deployment is Russia's first move of such warheads - shorter-range, less powerful nuclear weapons that could be used on the battlefield - outside Russia since the fall of the Soviet Union.
The United States has said it has no intention of altering its stance on strategic nuclear weapons in response to the deployment and has not seen any signs that Russia is preparing to use a nuclear weapon.
In May, Russia dismissed Biden's criticism of its plan to deploy tactical nuclear weapons in Belarus, saying the U.S. had for decades deployed such nuclear weapons in Europe.
The Russian deployment is being watched closely by the United States and its allies as well as by China, which has repeatedly cautioned against the use of nuclear weapons in the war in Ukraine.