Matthew Miller made the remark after Russian President Vladimir Putin warned on Thursday that Moscow had weapons to strike targets in the West, the Reuters news agency reported.
"It is not the first time we have seen irresponsible rhetoric from Vladimir Putin," Miller told reporters later in the day.
"It is no way for the leader of a nuclear-armed state to speak," he added. "We have communicated in the past privately and directly with Russia about the consequences of the use of a nuclear weapon. That said, we do not have any sign that Russia is preparing to use a nuclear weapon, and we will continue to monitor this carefully."
Putin warned NATO countries on Thursday that there was a risk of nuclear war if they sent their troops to fight in Ukraine.
Putin said in December that his country had no plans to attack the NATO military alliance, dismissing remarks by US President Joe Biden that Russia would attack a NATO country if it won the war in Ukraine.
Russia invaded Ukraine on February 24, 2022, starting the largest armed conflict in Europe since World War II.
Friday is day 737 of Russia’s war on Ukraine.
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Source: Reuters, IAR, PAP, state.gov