Japanese fishermen were out at sea attempting to trap more dolphins on Wednesday, campaigners said, after the bloody slaughter of dozens of the animals the previous day was hidden from view behind screens.

Clouds of blood drifted through the waters of the cove in Taiji on Tuesday as metal spikes were driven into the spinal columns of bottlenose dolphins that had been trapped for several days, environmentalists said.

Activists from the militant Sea Shepherd group, who are keeping vigil at the site in western Japan, said several dozen animals were killed behind specially-erected tarpaulin sheets.

Video footage from the group showed fishermen in wetsuits grappling with the dolphins as they herded them into the screened-off area to be butchered.

On Wednesday the hunters' boats were out on the ocean looking for more pods, the group said on its Twitter feed, but added that the pod they had initially been chasing had got away.

The group said on its Facebook page that 41 dolphins had been killed so far, and 52 had been removed from the cove. It says the fishermen sell the captive creatures to aquariums and dolphinariums, sometimes for six-figure dollar sums.

A further 130-140 had been driven back out to sea, the group said, adding that it believed many of them would die over the coming days from injuries sustained during captivity or from hunger.

The mass slaughter of the animals came to worldwide attention with the Oscar-winning 2009 film The Cove, which graphically showed the cull in the bay at Taiji.

Local officials say the hunt is an economic necessity for an area that has little else in the way of industry, and accuse campaigners of cultural insensitivity.

They insist they no longer use the bloody killing methods depicted in The Cove, but employ a more "humane" technique in which the dolphins' spinal cords are severed, causing instant death.

However, the hunt continues to provoke strong reactions, especially in the West, although Japan defends the practice as a traditional -- and legal -- fishing method that is vital to the local economy.

Criticism on Twitter from US ambassador to Japan Caroline Kennedy at the weekend, who said she was "deeply concerned by inhumaneness", was met with a curt response from Tokyo.

It said dolphins are an "important marine resource, which should be sustainably used based on scientific data".

While some hunt supporters in Japan have said they believed Kennedy's tweet might been loose talk from a first-time diplomat, Washington confirmed it reflected the official stance of the United States.

"The US does remain committed to the global moratorium on commercial whaling, and we are concerned with both the sustainability and the humaneness of the Japanese dolphin hunts," said State Department spokeswoman Marie Harf.

"We have been very clear that this is our position, and we remain concerned about it. And the ambassador was expressing our view that we’ve made public for a long time."

The United States has "discussed our concerns directly with Japan," she said.

Dolphins are outside the scope of the whaling moratorium. Japan has also faced intense international criticism for its annual hunting voyage to the Antarctic Ocean.

Tokyo says it conducts vital scientific research using a loophole in the moratorium, but makes no secret of the fact that the whales ultimately end up as food.

Australia wants Japan's annual whale hunt in the Southern Hemisphere summer to stop and has taken the matter to the UN's International Court of Justice. A decision is expected early this year.

On Wednesday it was revealed that Japan's whaling mothership, the Nisshin Maru, has received halal certification, proving that animals are slaughtered in accordance with Islamic law and can be eaten by Muslims.