Commercial whaling has been banned since 1986 under the a moratorium issued by the International Whaling Commission (IWC), however in Iceland the government regulates the hunting of whales having expressed reservations about its prohibition.
"While I can't entirely rule out the possibility that this is a hybrid, I don't see any characteristics that would suggest that", said Dr Phillip Clapham of the NOAA Alaska Fisheries Science Centre.
However, Iceland doesn't follow the worldwide moratorium on killing all whales, and will give permits for hunting fin whales, which they do not believe are threatened.
"There is nearly no possibility that an experienced observer would have misidentified it as anything else at sea". "We heard about this odd whale straight away and an employee reports that it's in many ways similar to a hybrid which has been brought to us quite a lot recently which is unusual. When you approach a blue whale, it's so distinct that you leave it alone".
If confirmed to be a blue whale, it would be the first detected harpooning in about 50 years.
"Whale 22 (documented by us on July 7th midnight / July 8th early morning) shows features of a blue whale (darker belly, all black baleen, bluish colour)", they wrote in a Facebook post.
And, according to the Whale and Dolphin Conservation, Icelandic whalers have killed four hybrid blue and fin whales over the years.
Whaling company Hvalur hf, which reportedly killed the whale, is allowed to hunt fin whales legally in Iceland.
Speaking to CNN, professor of biology Adam A.Peck at the University of Hawaii says that the whale hunted by Hvalur hf on Saturday night is a blue whale and not a hybrid blue/ fin whale as believed by Icelandic experts.
Sea Sheperd said it was the 22nd whale killed by Loftsson's outfit in the past three weeks, the others being endangered fin whales. Blue whales, the world's largest species, have been protected since 1966.
Like fin whales, they are listed by the International Union for Conservation of Nature as "endangered", with an estimated global population of between 10,000 and 25,000.
"This awful incident comes as Japan is rumoured to be planning an attempt to overturn the global moratorium on commercial whaling, and clearly speaks to how utterly inappropriate it is for countries to even contemplate allowing a large-scale return to this grossly inhumane and haphazard industry".
"This man must be stopped from ruthlessly violating worldwide conservation law and bringing such disrepute to the nation of Iceland". "We see them in the ocean".