A group of anti-whaling activists is chasing Japan's whaling fleet in the icy waters off Antarctica, as the country's annual Antarctic whale hunt gets under way.

Sea Shepherd says it caught up with all five of Japan's whaling vessels on Monday. The group released images of several dead whales lying on the deck of one ship.

Each whaling season, Sea Shepherd and Japan engage in clashes that occasionally turn aggressive in the remote Antarctic, with stink bombs, water cannons and ship collisions common features.

Japan is allowed to hunt the animals for scientific purposes under an exception to a 1986 ban on whaling. But opponents argue the scientific program is a cover for commercial whaling because whale meat not used for study is sold as food in Japan.