The Trump administration on Jan. 28 imposed sweeping sanctions on Venezuelan state-owned oil firm PDVSA, the toughest U.S. financial measure so far against Venezuelan President Nicolas Maduro.

The sanctions were announced to ratchet up pressure against Maduro and demonstrate U.S. support for Juan Guaido, the Venezuelan opposition leader who proclaimed himself interim president last week and was immediately recognized by the United States and a host of other countries.

“We have continued to expose the corruption of [Venezuelan President Nicolas] Maduro and his cronies and today’s action ensures they can no longer loot the assets of the Venezuelan people,” U.S. national security adviser John Bolton told reporters at a White House briefing.

PDVSA is OPEC nation Venezuela’s largest source of revenue and owns U.S. refining arm Citgo Petroleum, Venezuela's most important foreign asset.

U.S. officials said the sanctions on PDVSA were intended to prevent Maduro’s government from siphoning off funds from the oil company.

Until now, President Donald Trump had held off on targeting Venezuela’s vital oil sector. Washington has been mindful of the potential to deepen the hardship of the Venezuelan people and hurt U.S. companies and American consumers.