The U.S. sanctioned two foreign companies for allegedly delivering oil from Venezuela to Cuba, part of a continuing effort to step up pressure on the regime of President Nicolas Maduro.

The companies—linked to the delivery of oil during February and March aboard the tanker Despina Andrianna—are Ballito Bay Shipping Inc., based in Monrovia, Liberia and ProPer In Management Inc., based in Piraeus, Athens, the U.S. Treasury Department said in a statement. Ballito is the owner of the ship and ProPer is the operator, according to the statement.

Treasury said the sanctions show how the oil industry provides a “lifeline” to Venezuela’s government, led by Maduro, an autocrat under pressure from the U.S. to step down. The U.S. recognizes opposition chief Juan Guaido as the nation’s interim president.

“We are taking action against a vital source of the Maduro regime’s wealth,” Vice President Mike Pence said in prepared remarks on Friday. 

The U.S. also identified as “blocked property” 34 ships that belong to the Venezuela state-owned oil company PDVSA.