Environmental activist group Sea Shepherd claimed Monday that Japanese whalers attacked its vessels in the Southern Ocean, and accused the Australian government of "broken promises" to monitor whaling operations. Sea Shepherd said Japanese harpoon ships, the Yushin Maru and Yushin Maru 3, towed steel cables across the bow of its vessel the Bob Barker 11 times on Sunday in a bid to jam its propeller and rudder. It said that when the Bob Barker launched two small boats to defend the ship and cut the steel cables, a bamboo spear was thrown at crew members. Japan's fisheries agency said its ships were dragging cables with buoys as a warning to Sea Shepherd to stay away. "The cables were already there and they (Sea Shepherd) approached the cables on their own," an agency official told AFP, while denying any knowledge of a spear being thrown. "It was designed for them not to come near (the Japanese) ship." High-seas confrontations are common between Sea Shepherd and the Japanese, who hunt whales off Antarctica under a "scientific research" loophole in the moratorium on whaling. A 2010 collision resulted in the sinking of Sea Shepherd's speedboat Ady Gil. Sunday's incident was the third clash since the whaling season started earlier this year. No one was injured. "Each time we have located the Nisshin Maru (factory ship), the Sea Shepherd fleet has been attacked by the whalers in night-time ambushes," said Bob Barker's captain Peter Hammarstedt. The ship's helicopter located the Nisshin Maru early Sunday with a minke whale onboard while "slabs of whale meat were also photographed on the deck, along with the severed head of a recently butchered whale". The Bob Barker has nine Australians on board and Hammarstedt said he wrote a letter to Australian Environment Minister Greg Hunt last week complaining about a lack of action after the earlier assaults, which he said went unanswered. Hunt had initially promised to send a government ship to tail the warring groups during the annual hunt but opted instead for aerial surveillance. Sea Shepherd has previously described it as a "pretty cowardly" backdown to appease Japan due to ongoing free trade negotiations. Hunt's office said Monday a response had been sent to Sea Shepherd and defended the use of a plane rather than a ship. "For operational reasons, the use of a plane has been determined as the most effective means of monitoring activities in the Southern Ocean. The aircraft will be able to monitor activities over a large area," a spokesman said. The commercial hunting of whales is prohibited in the Southern Ocean Whale Sanctuary, which was designated by the International Whaling Commission in 1994, but Japan catches the animals there under a "scientific research" loophole. Australia has taken Japan to the International Court of Justice seeking to have its research whaling programme declared illegal, with a ruling due this year.