China on Thursday warned Japan against "playing with fire" in the contested waters of the South China Sea, after Tokyo announced it may patrol alongside the US in the region.
China also sent fighter planes for the first time over a strait near Japan on Monday as part of a group of more than 40 jets headed to train in the West Pacific.
The move followed remarks by Japanese Defence Minister Tomomi Inada this month that Tokyo would increase its engagement in the South China Sea through joint training with the US Navy, exercises with regional navies and capacity-building assistance to coastal nations.
The Chinese defence ministry said the aim of the announcement was "to mess up the South China Sea situation and try to gain interests from the troubled waters."
"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," ministry spokesman Yang Yujun told a regular press briefing.
Beijing asserts sovereignty over almost all of the South China Sea, dismissing rival partial claims from its Southeast Asian neighbours. It rejects any intervention by Japan in the waterway.
In recent months Japanese Prime Minister Shinzo Abe has criticised China for rejecting a July ruling by an international tribunal, which said Beijing's extensive claims to the waters had no legal basis.
Tokyo, a key US ally, is also strengthening defence ties with other countries in the disputed region. Japan and China are already at loggerheads over a longstanding territorial row in the East China Sea.
That dispute relates to uninhabited islets controlled by Japan known as the Senkakus in Japanese and the Diaoyus in Chinese.