Tensions are mounting between Russia, China, and Japan in the Pacific

Japan on Monday switched on a radar station in the East China Sea, giving it a permanent intelligence gathering post close to Taiwan and a group of islands disputed by Japan and China, drawing an angry response from Beijing.

The new Self Defence Force base on the island of Yonaguni 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 has raised concerns with its neighbors and in the West with its assertive claim to most of the South China Sea where the Philippines, Vietnam, Malaysia, Taiwan and Brunei have overlapping claims. Japan has long been mired in a territorial dispute with China over the East China Sea islands.

"Until yesterday, there was no coastal observation unit west of the main Okinawa island. It was a vacuum we needed to fill," said Daigo Shiomitsu, a Ground Self Defence Force lieutenant colonel who commands the new base on Yonaguni.

"It means we can keep watch on territory surrounding Japan and respond to all situations."

Shiomitsu on Monday attended a ceremony at the base with 160 military personnel and around 50 dignitaries. Construction of some buildings, which feature white walls and traditional Okinawan red-tiled roofs, is still unfinished.

The 30-sq-km (11-sq-mile) island is home to 1,500 people, who mostly raise cattle and grow sugar cane. The Self Defence Force contingent and family members will increase the population by a fifth.

"This radar station is going to irritate China," said Nozomu Yoshitomi, a professor at Nihon University and a retired major general in the Self Defence Force.

In addition to being a listening post, the facility could be used a base for military operations in the region, he added.

China's defense ministry, in a statement sent to Reuters about the radar station, vocing claims that the Diaoyu Islands were Chinese territories, and they oppose "provocative behavior" by the Japanese aimed at them.

"The activities of Chinese ships and aircraft in the relevant waters and airspace are completely appropriate and legal," the statement read.

The listening post fits into a wider military build-up along the island chain, which stretches 1,400 km (870 miles) from the Japanese mainland.

Policy makers last year told Reuters it was part of a strategy to keep China at bay in the Western Pacific as Beijing gains control of the South China Sea.

Toshi Yoshihara, a US Naval War College professor, said Yonaguni sits next to two potential flashpoints in Asia - Taiwan and the Senkaku/Diaoyu Islands. Yonaguni is only around 100 km (62 miles) east of Taiwan, near the edge of a controversial air defense identification zone set up by China in 2013.

"A network of overlapping radar sites along the island chain would boost Japan's ability to monitor the East China Sea," he added.

The radar installations also throw cold water on recent Chinese attempts to create a true blue water navy with access to shipping lanes and the ability to project naval power globally.

Over the next five years, Japan will increase its Self Defence Force in the East China Sea by about a fifth to almost 10,000 personnel, including missile batteries that will help Japan draw a defensive curtain along the island chain.

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The disputed islands are called the Northern Territories in Japan and Southern Kuriles in Russia.

Looming threat from Russia

At the same time that Japan seeks to hedge against Chinese military expansion in the East and South China Seas, Russian Defence Minister Sergei Shoigu said last week that Russia would study the possibility of building a naval base, and deploying advanced missile systems on what it calls the Kurile islands.

Japan claims a southern segment of the island chain, which was seized by Soviet troops at the end of World War Two. The territorial row has kept the two countries from signing a formal peace treaty since.

"We've informed the Russian side through a diplomatic channel that we are concerned about the comment by Defence Minister Shoigu" Chief Cabinet Secretary Yoshihide Suga told a regular news conference.

"We've also told them that if this led to the reinforcement of Russian military infrastructure on the Northern Territories, that would be incompatible with Japan's stance and regrettable."