(Reuters) - The United States on Monday imposed sanctions on four state governors in Venezuela as it ramped up pressure on President Nicholas Maduro for blocking humanitarian aid convoys.

The action coincides with U.S. Vice President Mike Pence attending a meeting of the regional Lima Group of nations in Bogota, where he said the United States was readying an additional $56 million in humanitarian aid. He called on Latin American allies to increase pressure on Maduro’s government.

“We call on all Lima Group nations to immediately freeze the assets of PDVSA. Secondly, transfer ownership of Venezuelan assets in your country from Maduro’s henchmen to President Guaido’s government,” Pence said, according to prepared remarks.

He also called on the countries to restrict visas for officials close to Maduro and to vote to recognize the representative of Juan Guaido, Venezuela’s self-declared interim president, at the Inter-American Development Bank.

The violence that erupted during the attempted delivery of much-needed food and medicine to economically devastated Venezuela has steeled the United States’ resolve to support opposition leader Guaido, Pence said.

Violent clashes with security forces over the opposition’s U.S.-backed attempt on Saturday to take aid into Venezuela left almost 300 wounded and at least three protesters dead near the Brazilian border.

The U.S. Treasury said it was imposing sanctions on Omar Jose Prieto, governor of the northwestern state of Zulia that borders Colombia; Ramon Alonso Carrizalez, governor of the state of Apure and a former defense minister; Jorge Luis Garcia, governor of Vargas state and former head of the army; and Rafael Alejandro Lacava, governor of Carabobo state and ally of Maduro.

“The illegitimate Maduro regime’s attempts to blockade international aid intended for the Venezuelan people are shameful, U.S. Treasury Secretary Steven Mnuchin said in a statement.

“Treasury is targeting four state governors aligned with former President Maduro for standing in the way of severely needed humanitarian assistance and prolonging the suffering of the Venezuelan people,” he added.

Governor Lacava in 2018 visited Washington as part of talks that paved the way for the release of Joshua Holt - an American man who was imprisoned in Venezuela for nearly two years. Lacava goes by the nickname “Dracula” in reference to his habit of doing late-night patrols and is known for off-the-cuff social media videos.