“If Japan wants to conduct any joint patrol or joint exercises in waters administered by China, it is just like playing with fire, and the Chinese military will not sit and watch,” said Yang Yujun, the Chinese Defense Ministry spokesman, on Thursday.
The Japanese Defense Ministry had announced earlier this month that Tokyo would increase its engagement in the South China Sea through joint training cruises with the US Navy, capacity-building assistance to coastal countries and exercises with regional navies.
On Monday, China sent more than 40 fighter jets to train over a strait near Japan for the first time.
Japan, a key US ally, is boosting defense ties with the Philippines and other Southeast Asian countries that have their own territorial disputes with Beijing in the South China Sea.
China asserts sovereignty over almost all of the strategically vital waters amid rival claims from its Southeast Asian neighbors. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have overlapping claims in the resource-rich sea.
China also claims the uninhabited East China Sea islets known as Senkaku in Japan and Diaoyu in China, and occasionally sends coastguard vessels close to them, a move that angers Tokyo.
The South China Sea waters are believed to sit atop vast reserves of oil and gas. The dispute usually draws in trans-regional countries, particularly the US.
China accuses the US of interfering in the regional issues and deliberately stirring up tensions in the South China Sea. Washington, in turn, accuses Beijing of carrying out what it calls a land reclamation program in the South China Sea by building artificial islands in the disputed areas.