Heat turned up on China-Japan relationship with maritime disputes over islands and alliances

Tension between Asia's two biggest economies is heating up as both countries build up their militaries and boost maritime activity.

On Tuesday, Japan again criticised China for sending ships into waters around the disputed Senkaku Islands, which China calls the Diaoyu Islands, in the East China Sea. Tokyo said it was the 11th time this year "Chinese government ships have been spotted entering Japanese territorial waters off the islands".

But compared with the long-running island dispute, in which Beijing mostly sends coastguard vessels, separate Chinese warship activities near Japan are more alarming from Tokyo's perspective.

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Three Chinese warships - the Lhasa, a Type 55 guided-missile destroyer, destroyer Chengdu and replenishment ship Dongpinghu - were spotted on Tuesday in the Izu Islands south of Tokyo, according to Japan's defence ministry.

The Chinese warships had been operating in partnership with five Russian warships since June 12, the ministry said, but there was no confirmation from Russia and China they were coordinating with each other.

Japan was then prompted to send a Maritime Self-Defence Force flotilla to 11 Indo-Pacific countries as part of joint naval exercises with the US and its allies to counter a more assertive China, according to Kyodo news agency.

China has been sending warships to waters near Japan since late last year. In November, a Chinese naval ship sailed in Japan's waters off its southwestern prefecture of Kagoshima, the first such movement since July 2017, Kyoto reported, citing Japan's defence ministry.

During exercises in May, the Chinese aircraft carrier Liaoning launched fighters near Japan and the carrier was later shadowed by Japan's Izumo helicopter destroyer in the western Pacific.

James Bosbotinis, a specialist in defence and international affairs, said Chinese warships were bound to sail deeper into the ocean as the PLA Navy (PLAN) became more advanced.

"As the PLAN develops it is venturing further afield and to access the northern Pacific, for example, requires transiting through the Sea of Japan and the Tsugaru Strait," he said. "There is also a lot of signalling, though, on the part of Beijing with its naval deployments around Japan, and driven by Japan's position on Taiwan, cooperation with the US and a perception in China that Japan is working with the US to contain it."