Japan has summoned senior diplomats to complain about an activist attack on its Antarctic whaling fleet using mild acid made from rotten butter.
The Sea Shepherd group carried out the attack from their boat, the Steve Irwin, which last docked in Australia and is registered in the Netherlands.
Tokyo officials have protested to the ambassadors of both those nations.
They have also pledged to raise the issue at this week’s meeting of the International Whaling Commission (IWC).
“It’s not permissible to use violence to try to force through one’s opinion,” AFP news agency quoted Foreign Minister Masahiko Komura as saying.
The IWC will meet in London for a three-day informal gathering in an attempt to break the deadlock between pro-whaling and anti-whaling nations.
Activists and whalers have clashed several times since last November, when the Japanese fleet set sail.
Protesters from Greenpeace and the more radical Sea Shepherd group have tracked the fleet in their own vessels.
Tokyo officials said Monday’s attack lightly injured two crew members and two Japanese coast guard officers on one of the whaling ships.
But Sea Shepherd denied anyone had been hurt.
Australia is one of the most vehement anti-whaling countries, and has taken Japan to task several times, but Canberra officials condemned Monday’s attack.
Japan had planned to kill up to 900 minke whales and 50 fin whales during the expedition.
Tokyo says it carries out whaling for scientific research, but critics say the same data can be collected without killing the animals.