Amid already tense relations between China and Japan over territorial disputes, a new controversy arose this week when three Chinese ships — including an armed ex-navy frigate — sailed within the 12-mile exclusion zone of islands in the East China Sea that Japan has claimed, a Japanese official told Bloomberg.

The ships’ deployment is thought to be a possible attempt at distracting Japan from territorial disputes in the South China Sea, and a challenge to Japan’s claims over the East China Sea islands.

“China doesn’t want Japan to meddle in the South China Sea,” Giulio Pugliese, assistant professor at the University of Heidelberg’s Institute of Chinese Studies, told Bloomberg. “China’s engagement in the East China Sea and its constabulary build-up remind Japan of the risks of stretching out its naval presence to distant Southeast Asian waters.”

Japanese Prime Minister Shinzo Abe and Chinese President Xi Jinping have met twice in the last year and have pledged to reduce the risk of conflict over the islands, but tensions have only escalated in recent weeks after sightings of armed Chinese coast guard ships were reported entering waters close to the disputed Senkaku Island chain. Experts say the incidents are only likely to escalate from here.

“In the foreseeable future, more and more of the China coast guard ships entering the waters off the isles, or even engaging in standoffs with Japanese coast guard, will be newer, larger and armed vessels,” said Collin Koh Swee Lean, an associate research fellow at Singapore’s S. Rajaratnam School of International Studies. “This ought to be worrisome insofar as the Japanese come to realize that the China coast guard is fast bridging its capability gap, eroding the qualitative advantages Japanese coast guard has long held over China’s maritime agencies.”

The U.S. has pushed Japan recently to take an aggressive stance against China’s artificial island building in the area. Japan has responded by increasing military personnel stationed in the East China Sea and preparing to deploy anti-ship and anti-aircraft missile batteries along its islands.