Japan has activated its first marine unit since World War Two, trained to counter invaders occupying Japanese islands along the edge of the East China Sea that Tokyo fears are vulnerable to attack by China.

In a ceremony held at a military base on the southwest island of Kyushu, about 1,500 members of the Amphibious Rapid Deployment Brigade wearing camouflage lined up outside amid cold, windy weather.

“Given the increasingly difficult defence and security situation surrounding Japan, defence of our islands has become a critical mandate,” Tomohiro Yamamoto, vice-defence minister, said in a speech.

The troops conducted a 20-minute mock public exercise, enacting a scenario that saw them recapturing a remote island from invaders.

The formation of the Japanese marine brigade is controversial because amphibious units can project military force and could, critics warn, be used to threaten Japan’s neighbours.

In its post World War Two constitution Japan renounced the right to wage war.

The brigade is the latest component of a growing marine force that includes helicopter carriers, amphibious ships, Osprey tilt-rotor troop carriers and amphibious assault vehicles, meant to deter China as it pushes for easier access to the Western Pacific.

China, which dominates the South China Sea, is outpacing Japan in defence spending.

In 2018, Beijing which claims a group of uninhabited islets in the East China Sea controlled by Tokyo, will spend 1.11 trillion yuan ($US176.56 billion) on its armed forces, more than three times as much as Japan.

The activation of the 2,100 strong ARDB takes Japan a step closer to creating a force similar to a US Marine Expeditionary Unit (MEU) able to plan and execute operations at sea far from its home base.

Japanese military planners are already mulling additions. Its Air Self Defence Force (ASDF) wants to acquire F-35Bs to operate from its Izumo and Ise helicopter carriers, or from islands along the East China Sea, according to sources.

-ABC