China strongly warned a high-level Japanese official around late June that Tokyo should not send its Self-Defense Forces to join US operations that test the freedom of navigation in the disputed South China Sea, diplomatic sources said. 
Japan will "cross a red line" if SDF vessels take part in US freedom of navigation operations, Chinese Ambassador to Japan Cheng Yonghua told the Japanese official, the sources said, adding that the ambassador even hinted at military action if Japan crosses the line. 
The warning was issued just as an international tribunal was about to rule on conflicting claims in those waters, Japan's News Agency (Kyodo) reported. 

Source : QNA