Japanese whaling vessels have embarked on a mission to kill 333 minke whales in the Antarctic Ocean, once again claiming that the practice is for research purposes. It comes amid international calls for the country to end the controversial killings.

Two vessels set sail from Shimonoseki, Yamaguchi Prefecture on Thursday, according to the Fisheries Agency. They will join the 8,145-ton mother ship ‘Nisshin Maru’ and two other ships departing from other parts of Japan before heading south, Kyodo News reported.

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The Fisheries Agency claims the practice is aimed at studying whale behavior and biology, saying it is carried out "to devise more precise calculation methods for a sustainable catch limit for Antarctic minke whales, as well as to study the ecosystem of the Antarctic waters." Activists, including the anti-whaling group Sea Shepherd, have called Tokyo's bluff, however, saying the so-called "research" is merely a cover for commercial whaling, as meat from the captured whales is later sold.

Japan is a signatory to the International Whaling Commission's (IWC) moratorium on hunting. However, claiming the practice is for research purposes allows the country to exploit a loophole in the ban.

The current whaling program was devised after the United Nations' International Court of Justice (ICJ) ordered Tokyo to end its hunt in 2014, saying the project did not meet conventional scientific standards. Japan then canceled its 2014-2015 hunt, only to resume the following year under a different program, claiming that the new project had genuine scientific purposes.

Japanese whalers have clashed in open waters with animal-rights campaigners in the past, particularly Sea Shepherd. However, the organization announced in August that it would not be facing off with whalers this year, as it "cannot compete with their military-grade technology."

Still, the Fisheries Agency said it is taking measures to ensure the safety of its whalers on the current mission. It has also urged countries that provide ports to Sea Shepherd ships to cooperate, AFP reported.