"As the Korean coastline is not very long, it is efficient to use MGO [marine gasoil]," a bunker trader in South Korea said.

That followed complaints from ship supply companies that they were being prevented from delivering to ships in some ports.

"Only staff on essential duties should be nominated and permitted to travel to offshore/remote locations."

"Some vessel operators think it is stressful for crew to stay onboard for longer and better for them to go home. I disagree," Jensen said.

Crew and passengers on short-term visits would also not be allowed to disembark in Singapore for the time being, the MPA said.

"MOFCOM has informed us that fuel oil exports will be restricted by quota soon," a Beijing-based source said.