Icelandic whalers appear to have killed an endangered blue whale before chopping it up to be eaten as a delicacy in Japan.

Photos of the massive mammal, which can grow to 33 metres long, were posted online by conservation groups claiming it was slain by Kristján Loftsson's whaling company, the Daily Mail reports.

The huge carcass was seen being hauled in to port by the Hvalur 8 ship while tied to the side of the vessel before being dragged on to the dock on Saturday evening.

Crew members then took turns posing for photo straddling its back, having needed run-ups to clamber on top of the world's biggest animal, video from the scene showed.

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Blue whales were almost hunted to extinction last century and there are only 10,000 to 25,000 left alive. One has not been slaughtered for more than 50 years.

Sea Sheperd said it was the 22nd whale killed by Loftsson's outfit in the past three weeks, the others being endangered fin whales.

"Loftsson ordered his crew to butcher the whale just like it was another Fin whale,' the anti-whaling group claimed.

"The meat, skin, blubber and bone all now mixed in with the fin whales previously caught, which will make it difficult or impossible to locate during potential inspections by the authorities."

Later photos and video showed the whale being butchered, and activists said the meat would be sold to Japan where whales are still eaten in restaurants.

Loftsson is allowed by the Icelandic Government to hunt fun whales, despite the global moratorium on whaling, but killing blue whales is illegal everywhere.

Multiple experts who saw the photos said the whale was almost certainly a blue whale, going by its colour, pattern, and fin and tail shape.

It has all the characteristics of a blue whale; given that - notably the coloration pattern,' Dr Phillip Clapham from NOAA Alaska Fisheries Science Centre said.

"There is almost no possibility that an experienced observer would have misidentified it as anything else at sea."

Humane Society International senior marine scientist Mark Simmonds also believed it was a juvenile blue whale or a rare fin whale-blue whale hybrid.

Sea Shepherd UK chief operating officer Robert Read demanded DNA samples be taken from the Hvalur 8's equipment, meat stocks, and storage to prove it illegally killed a blue whale.

"This man must be stopped from ruthlessly violating international conservation law and bringing such disrepute to the nation of Iceland. There can be no legal justification for this crime."