The US Treasury Department on Tuesday said it had sanctioned four shipping firms for transporting Venezuelan oil, the latest escalation in Washington's effort to oust socialist President Nicolas Maduro by cutting off the OPEC nation's crude exports.

Marshall Islands-based Afranav Maritime Ltd, Adamant Maritime Ltd and Sanibel Shiptrade Ltd, as well as Greece-based Seacomber Ltd, all own tankers that lifted Venezuelan oil between February and April of this year, the Treasury Department said.

"These companies are transporting oil that was effectively stolen from the Venezuelan people," Secretary of State Mike Pompeo said in a statement.

Washington sanctioned Venezuelan state-run oil company Petroleos de Venezuela last January, shortly after the United States and dozens of other countries declared Maduro a usurper who rigged his 2018 re-election.

Maduro blames the sanctions for Venezuela's woes and accuses Washington of seeking to oust him in order to control the country's vast crude reserves.

Neither Venezuela's oil ministry nor PDVSA immediately responded to requests for comment. Afranav did not immediately respond to a request for comment, while the other three companies could not be reached for comment.

But Maduro remains in power, which some US officials privately say has been a source of frustration for President Donald Trump.

Tuesday's sanctions come after Washington in February and March sanctioned two units of Russia's Rosneft, which became the main intermediary of Venezuelan crude in 2019. The units stopped lifting Venezuelan crude in March.

The FBI is also probing several Mexican and European companies that are allegedly involved in trading Venezuelan oil. One of those companies, Libre Abordo, said this week it was bankrupt.

Treasury also designated four tankers owned by the companies as blocked property. Those tankers had been used by Rosneft, Libre Abordo and a related Mexican firm - Schlager Business Group - to transport Venezuelan oil this year, according to PDVSA documents.

Treasury also designated four tankers owned by the companies as blocked property. Those tankers had been chartered this year by the Rosneft units, Libre Abordo and a related Mexican firm - Schlager Business Group - to transport Venezuelan oil, according to PDVSA documents.