US ambassador to Japan Caroline Kennedy has tweeted her concern at the "inhumaneness" of a Japanese village's traditional dolphin hunt.

"Deeply concerned by inhumaneness of drive hunt dolphin killing. USG (US Government) opposes drive hunt fisheries," she tweeted on January 17.

Every year the fishermen of Taiji corral hundreds of dolphins in a secluded bay, select a few dozen for sale to aquariums and marine parks and stab the rest to death for meat.

This year fishermen and divers in the village have caught at least 25 dolphins in a process to select captives before the mass slaughter, environmentalists said Saturday.

On Sunday a fishing industry official in Taiji said some dolphins had already been slaughtered, although he did not disclose the number.

Activists from the militant environmental group Sea Shepherd have streamed live footage of the dolphin capture in Taiji, which drew worldwide attention in 2010 when it became the subject of the Academy Award-winning documentary "The Cove".

The town's fishermen defend the hunt as a cultural tradition, and "The Cove" was met by protests from right-wing activists when it was screened in Japan in 2010.

The fishing industry official, who declined to be identified, said dolphins died almost instantaneously when their spinal cords were cut.

There was no bloodshed and the sea did not turn red, unlike scenes in "The Cove", he said.

"We've got our lives. We can't simply nod (to protests) and end centuries of our tradition... If you want to talk about cruelty, you couldn't eat cows, pigs or any other living creatures," he told AFP by telephone.

But Sea Shepherd in a statement Saturday condemned what it called a brutal practice.

"Those taken captive are forced to watch as the remaining members of their family are brutally killed for human consumption," it said.

Kennedy, the lone surviving child of assassinated president John F. Kennedy, took up the diplomatic post late last year.