Anti-whaling activist group Sea Shepherd has claimed a whaling company in Iceland killed an endangered blue whale.

"Sea Shepherd volunteers monitoring the Hvalur hf whaling station in Hvalfjordur Iceland have documented the slaughter of an endangered Blue whale on the night of July 7th," the group said in a statement.

Photos of the whale provided by Icelandic-based anti-whaling group Hard to Port show the large animal being towed into a port and crew members standing alongside it.

However experts believe the whale could be a blue whale-fin whale hybrid. Hard to Port also agreed the whale could be a hybrid, according to ABC News.

Hard to Port

Whaling company Hvalur hf, which reportedly killed the whale, is allowed to hunt fin whales legally in Iceland.

Gisli Arnor Vikingsson from the Iceland Marine Institute said he believed the animal was a blue whale–fin whale hybrid.

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"Photographs point to the fact that it's a hybrid whale and we're almost certain that it is one, but we can't be sure until autumn when we get it DNA tested," he said, according to ABC News.

A blue whale - the largest animal to have ever lived - has not been deliberately caught since 1978, according to the International Union for the Conservation of Nature (IUCN).

Iceland resumed commercial whaling in 2006. Although an international moratorium has placed catch limits at zero for blue whales, Iceland, Norway and the Russian Federation all objected to that provision.

During this year's season, Sea Shepherd said Hvalur hf had killed 22 whales.