Japanese fishermen on Friday killed the first dolphins of the season in a controversial annual hunt that attracted global attention after it was featured in the Oscar-winning 2009 documentary "The Cove".

An official with the local fishing union in the small town of Taiji said boats left early Friday morning and trapped a dozen dolphins.

"They have already been killed," he added.

During the six-month hunt, people from the southwestern town corral hundreds of the mammals into a secluded bay and butcher them, turning the water crimson red.

The scene was featured in the documentary, drawing unwanted attention to the little coastal community.

Environmental campaigners visit the town every year to watch the gruesome event and authorities have boosted their presence to prevent any clashes between locals and activists.

But the Taiji officials said only about half a dozen activists were there to see the killing and there were no altercations.

"It was quiet this year," he added.

Activists from the group Sea Shepherd criticised the hunt online.

"Now in Taiji, Japan: a pod of 10-15 Risso's dolphins is being slaughtered in the cove," the group said on its Facebook page.

The town kicked off its hunt last week, but bad weather hampered efforts to trap the dolphins, officials said earlier.

Defenders of the hunt say it is a tradition and point out that the animals are not endangered, a position echoed by the Japanese government.

Some of the dolphins are sold to aquariums.

Increasing the pressure on Japan, the World Association of Zoos and Aquariums (WAZA) suspended its Japanese chapter in April for using dolphins caught in Taiji.

The Japanese chapter later decided to stop buying dolphins from Taiji.

Last week, Taiji's official website was hacked into, leaving it temporarily inaccessible.

Anonymous, a loosely connected hacking collective, has listed 21 Japanese government websites as targets for cyber-attacks as part of a bid to end the annual hunt.

But police said they have yet to connect the group to the attack on the Taiji website.