Specifically, Fortune along with the Forest, Petunia, Faxon and Clavel are carrying fuel from Bandar Abbas to Puerto Cabello, as Venezuelan state media reported.

Although Venezuelan PDVSA was playing a crucial role in producing crude oil, years of mismanagement and sanctions have greatly reduced Venezuela's refining capacity, and fuel shortages are routine.

In efforts to offset shortages, the government of Venezuelan President Nicolas Maduro has ordered fuel from another sanctioned oil producer - the Islamic Republic of Iran.

Yet, US sanctions have led to Iran's oil exports plummeted, while longtime customers have turned to other suppliers. That trend is expected to accelerate now that oil costs less in general, leaving a smaller price advantage for buyers who purchase riskier but cheaper Iranian crude. Fuel exports to Venezuela - which has nothing to lose from sanctions - give Iran another much-needed income stream.

Both Venezuela and Iran have warned the US not to interfere with the shipment.