Hong Kong Airlines is under pressure to stop its live dolphin cargo business, with an online petition alleging that the mammals are transported in "flying coffins".

The dolphins are believed to have come from the Japanese town of Taiji, the scene of an annual dolphin slaughter.

The China Daily newspaper reported about the delivery of five dolphins from Japan to Vietnam, citing a Hong Kong Airlines memo praising the delivery for earning the company more than $100,000.

The report included a photo showing the dolphins lying in shallow, narrow containers inside the cargo plane.

Since then more than 2,900 people have signed an online petition demanding Hong Kong Airlines end the flights.

"Five Taiji dolphins were transported via cargo flight in 'flying coffins' on January 16, 2012. They spent at least seven hours in this cruel confinement," the petition reads.

"Dolphins are neither cargo, nor commerce, nor entertainment."

The group behind the petition has previously run campaigns against the dolphin slaughter in Taiji.

Hong Kong Airlines said it adhered to government rules and International Air Transport Association regulations on live animal transportation.

"Hong Kong Airlines is fully committed to the protection of animal welfare," it said in a statement.

"No dolphin suffered or (was) injured during this shipment."

It added that it was "totally unaware of the complexities" surrounding the "dark side of the dolphin story", and thanked animal welfare groups for their input.

ABC/AFP