Japan has started to construct a coastal surveillance unit near a string of islands at the centre of a bitter territorial dispute with China.
Radar equipment will be placed on Yonaguni island to monitor ships and aircraft in the East China Sea, the Kyodo news agency said on Saturday.
The Ground Self Defence Force (SDF) surveillance unit, comprising around 150 personnel, will be deployed on Yonaguni by the end of March 2016, the news agency said, citing Japan's defence ministry.
"It's very important to take a solid surveillance posture on remote islands," said Itsunori Onodera, Japan's defence minister, after attending the ground breaking ceremony.
The island lies around 150km southwest of the Tokyo-controlled Senkakus, which Beijing claims and calls the Diaoyus.
On Saturday, some Yonaguni residents opposed to the new surveillance unit scuffled with officials connected to the defence ministry.
Locals are concerned the island could become a target in any future conflict between Japan and China.
The surveillance unit will "fill a void of SDF presence" in Japan's remote southwestern islands, Onodera said.
Chinese vessels and aircraft have regularly approached the disputed East China Sea archipelago - thought to harbour vast natural resources - after Japan nationalised some of the islands in September 2012, setting off the latest spate of incidents in a long-running territorial row.
The ceremony comes at a time when Japanese Prime Minister Shinzo Abe is pushing to reconfigure Tokyo's role on the international stage, specifically that of its armed forces.
Abe wants to reinterpret a law to allow Japanese troops to take up arms to defend an ally under attack, so-called collective self-defence.
Beijing has sought to paint Abe's moves as a dangerous slide back towards Japan's militarism of the last century.