Hit by U.S. sanctions on Monday, Venezuela's state-run oil company, PDVSA, ordered customers with tankers waiting to load Venezuelan crude bound for the United States, to pay for the cargo before they depart.
Venezuela exports about 500,000 barrels per day of crude to the United States.
Citgo, Valero Energy Corp and Chevron Corp are the three largest buyers of Venezuelan crude in the United States.
U.S. President Donald Trump's administration, in a move aimed at limiting embattled President Nicolas Maduro's access to oil revenues, stopped PDVSA from collecting the proceeds of oil exported to U.S. customers, while freezing assets of PDVSA's units, including U.S. refining arm Citgo Petroleum Corp.
Maduro said earlier, his government would start legal action to protect Citgo.
He did not mention if PDVSA planned changes to its exports.