Japanese ships reportedly had a brush with Chinese vessels in already troubled waters, reflecting that seas surrounding the disputed Senkaku islands will unlikely calm down for some time.

Chinese surveillance ships were reportedly seen approaching waters near the islands, displaying a warning that read: “You are in waters administered by the People’s Republic of China. You are already breaching the law. Move way immediately.” Japan’s coastguard said it responded by demanding the Chinese ships leave.

China’s State Oceanic Administration confirmed that four of its patrol boats “carried out expulsion measures” against Japan Coast Guard ships “illegally” operating in the area, AP reports.

The confrontation is a notch up from previous Chinese patrols around the islands, before Beijing announced a territorial baseline defining its claim on the disputed islands.

“Chinese government vessels did not chase Japanese boats out of the islands’ territorial waters in the past, as these waters were an area controlled by the Japanese coastguard,” said Li Guoqiang, an expert on border issues at the Chinese Academy of Social Sciences. “But the situation changed when we created a legal basis for enforcing our claim by announcing the territorial baseline for the islands in September.”

Tensions have steadily mounted since Tokyo announced the nationalization of some of the disputed islands last month.