About a dozen tankers loaded with Venezuelan oil and fuel have left the country's territorial waters in recent days, turning off their transponders, according to the TankerTrackers.com monitoring service, Reuters reports.

They appear to have violated a US sanction imposed before the arrest of Venezuelan President Nicolas Maduro. As a result of that sanction, oil exports from Venezuela have been virtually paralyzed.

All of the identified ships that left Venezuela are under US sanctions.

Another group of ships, also under sanctions, left empty after unloading imported cargo or completing domestic shipments.