WASHINGTON — The Treasury Department imposed financial sanctions on a host of current and former senior Venezuelan officials on Wednesday and threatened to take more stringent action if President Nicolás Maduro proceeds with plans for a constituent assembly on Sunday that critics consider a danger to democracy.

Steven T. Mnuchin, the Treasury secretary, ordered assets in the United States frozen for 13 well-connected Venezuelan figures and barred Americans from doing business with them. Among those targeted by the administration were the interior minister and heads of the army, police and national guard, as well as government officials involved in the coming assembly.

“As President Trump has made clear, the United States will not ignore the Maduro regime’s ongoing efforts to undermine democracy, freedom and the rule of law,” Mr. Mnuchin said in a statement. “As our sanctions demonstrate, the United States is standing by the Venezuelan people in their quest to restore their country to a full and prosperous democracy.”

In a conference call with reporters, administration officials urged Mr. Maduro to cancel the Sunday assembly or face tougher actions. Among the possible options could be measures targeting oil sales. “Anyone elected to the National Constituent Assembly should know that their role in undermining democratic processes and institutions in Venezuela could expose them to potential U.S. sanctions,” Mr. Mnuchin added in his statement.