China's Defence Ministry accused Japan on Thursday of hypocrisy by opening a new base close to Taiwan and a group of islands disputed between Japan and China in the East China Sea at the same time as criticizing China's actions in the South China Sea.
Japan this week switched on a radar station on Yonaguni, giving it a permanent intelligence gathering post in a strategic location.
The new Self Defence Force base on the island is at the western extreme of a string of Japanese islands in the East China Sea, 150 km (90 miles) south of the disputed islands known as the Senkaku islands in Japan and the Diaoyu in China.
China's Defence Ministry, which has already expressed anger at the move, said Japan's armed forces had been "very active" of late, and Japan had also been decrying China's "so-called militarization" in the South China Sea.
"Today we can see what Japan is really up to on Yonaguni island, which is only 100 km from China's Taiwan and the Diaoyu Islands," ministry spokesman Yang Yujun told a monthly news briefing. China claims self-ruled Taiwan as its own.
"We've noticed media reports that in the past this island had just two policemen and only 10 rounds of ammunition," he added, saying Japan would now be basing troops there.
"Japan always criticizes others, so how can it explain its own actions?"
China lays claim to most of the South China Sea where the Philippines, Vietnam, Malaysia, Taiwan and Brunei also have claims. Japan has long been mired in a territorial dispute with China over the East China Sea islands.
Japan likes "making a scene" by talking about freedom of navigation, but the South China Sea is a wide and expansive place, unlike Yonaguni's location at a narrow choke point, Yang said.
"What should we call this increase in military deployment surrounding this narrow international channel which connects the East China Sea to the Western Pacific?" he added.
The U.S. Navy has carried out freedom of navigation exercises, sailing near disputed islands in the South China Sea to underscore its rights to operate in the seas.
Those patrols, and reports that China is deploying advanced missiles, fighters and radar equipment on islands there, have led Washington and Beijing to trade accusations of militarizing the region.
The South China Sea is one of the world's freest and safest shipping lanes, Chinese Foreign Minister Wang Yi said earlier in March.