Russia has previously sent an oil tanker to Cuba. It reached Cuba earlier this week, despite the US blockade.
"A tanker from Russia has broken through the US blockade. A second ship is now being loaded. We will not leave Cuba in the lurch," Russian Energy Minister Sergei Tsiviljov said on Thursday, according to state-run Russian media.
Cuba is severely affected by acute fuel shortages and widespread power outages since oil imports have largely ceased.
A crucial reason is the US decision to isolate Cuba after the US military removed Venezuela's former president, Nicolás Maduro, and then stopped oil deliveries from Venezuela to Cuba.
