Nuland said this during the Kiev Security Forum.
According to the US Undersecretary of State, Washington is "decisive in its message to Moscow."
"If they take steps to destabilize Ukraine and use their forces for aggression against Ukraine, then there will be serious economic steps and sanctions that have not yet been in the past," RIA Novosti quoted her as saying.
Earlier, US Secretary of State Anthony Blinken said that Washington would be ready to impose large-scale economic sanctions against Moscow in the event of an allegedly possible Russian invasion of Ukraine.
Russian Foreign Minister Sergei Lavrov announced the attempts of NATO states to turn Ukraine into an "anti-Russia".
The official representative of the Ministry of Foreign Affairs of Russia, Maria Zakharova, said at a briefing that, according to some sources, Kiev had drawn up to 125 thousand troops to Donbass.
She noted that this is half of the composition of the Ukrainian army.
The Kremlin said that accusations of aggression on Russia could hide an attempt by Kiev to resolve the Donbass issue by force.