"The United States will not stand by as Venezuela crumbles. If the Maduro regime imposes its Constituent Assembly on July 30, the United States will take strong and swift economic actions," Trump said.
Maduro's foes are demanding a presidential election and want to stop the leftist leader's plan to create the Constituent Assembly, which would have the power to rewrite the constitution and annul the opposition-led legislature.
On Sunday, 98 percent of opposition supporters in an unofficial vote rejected the proposed assembly.
Maduro insists opposition leaders are U.S. pawns intent on sabotaging the economy and bringing him down through violence as part of an international right-wing conspiracy led by Washington and fanned by private domestic and foreign media.
Senior White House officials told Reuters last month the Trump administration was considering sanctions on Venezuela's vital energy sector, including state oil company PDVSA, a major escalation in U.S. efforts to pressure the country's government amid a crackdown on the opposition.
The idea of striking at the core of Venezuela's economy, which relies on oil for some 95 percent of export revenues, has been discussed at high levels of the administration as part of a wide-ranging review of U.S. options.