Japanese whaling vessels left port Thursday for an annual hunting voyage in the Antarctic, this time to kill 333 minke whales, despite international calls to stop the practice.

The fisheries agency said a group of five ships, headed by the 8,145-tonne mother ship Nisshin Maru, will conduct the hunt until March to study whale behaviour and biology.

The voyage has been carried out since 2015 "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", the ministry said in a statement.

Japan is a signatory to the International Whaling Commission's (IWC) moratorium on hunting, but exploits a loophole that allows whales to be killed in the name of scientific research.

In 2014, the United Nations' International Court of Justice (ICJ) ordered Tokyo to end its regular hunt in the Antarctic waters, saying the project did not meet conventional scientific standards.

Japan cancelled its 2014-15 hunt, only to resume it the following year under a new programme -- saying the fresh plan had genuine scientific value.

Tokyo says it is trying to prove the whale population is large enough to sustain a return to commercial hunting for a traditional source of food.

Japanese whalers have in the past clashed on the high seas with animal rights campaigners, particularly Sea Shepherd.

The fisheries agency said it was taking measures to ensure the safety of its whalers and urged countries that provide ports to Sea Shepherd ships to cooperate.