"The United States considers the Venezuelan National Constituent Assembly the illegitimate product of a flawed process designed by the Maduro dictatorship to further its assault on democracy," Nauert said Thursday.
She added that the process of was "rigged from the start" and designed to fill the assembly with loyalists to President Nicolas Maduro.
"The United States will not recognize the National Constituent Assembly," she said, adding that the United States was evaluating all of its policy options.
Venezuela, mired in a deep economic crisis caused by the fall of oil prices, has been facing mass protests since spring. In May, Maduro announced his decision to convene the National Constituent Assembly, a move that was regarded by the opposition as an attempted coup.
The announcement fueled further escalation of tensions, with violent protests resulting in numerous casualties.
The Constituent Assembly is expected to convene for its first session on Friday.
