Upsetting its neighbours and Western countries because of its assertive claim to most of the South China Sea, China has accused Japan of stirring things up in the region for its own purposes.
China on Thursday warned Japan not to "play with fire" by stepping up activity in the disputed South China Sea through joint training patrols with the United States.
China's Defence Ministry said it would not sit watching from the sidelines as Japan is boosting its presence in the region.
It accused Tokyo of stirring things up in the South China Sea for its own purposes.
"We must solemnly tell Japan this is a miscalculation. If Japan wants to have joint patrols or drills in waters under Chinese jurisdiction this really is playing with fire," Chinese Defence Ministry spokesman Yang Yujun told a monthly news briefing.
"China's military will not sit idly by."
China claims most of the energy-rich waters of the South China Sea, through which about $5 trillion in ship-borne trade passes every year. But neighbours Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims to parts of the waters.
Japan has also been struggling with China over the East China Sea islands for a long time, and Tokyo is strengthening its ties in the region, in particular with the Philippines and Vietnam.
Ties between Asia's two largest economies have long been overshadowed by arguments over their painful wartime history and a territorial spat in the East China Sea, among other issues.