- China Coast Guard vessels sailed into Japanese waters, Japan said
- The countries are in a power struggle over the disputed island chain
The four Chinese boats were near an island chain that both countries claim as their own, a Japan Coast Guard spokesman said.
Japan's Foreign Ministry lodged a protest with the Chinese Embassy in Tokyo over Saturday's incident, a foreign ministry spokeswoman said.
Japan controls the islands, calling them Senkaku; China claims them as part of its territory and calls them the Diaoyu islands.
The Chinese boats were in Japan's territorial waters for about two hours before leaving at the request of a Japanese coast guard vessel, the spokesman for Japan Coast Guard's 11th Regional Command Headquarters told CNN.
Centuries-old dispute heats up
It was the 16th time this year that Chinese vessels have entered Japan's territorial waters, the Japanese official said.
warplanes last month after Chinese ships and a drone ventured into the same area, Japanese Defense Ministry officials said at the time. It marked the first time Beijing had used an unmanned aircraft in the island dispute.
The islands are uninhabited, but their ownership would allow for exclusive oil, mineral and fishing rights in the surrounding waters.
Though claims to the island chain date back centuries, the most recent round of tensions began in 2012, when Japan nationalized the islands to ward off a planned sale to Tokyo's then-governor, a hardline nationalist apparently hoping to develop the islands.
The plan sparked massive anti-Japanese protests across China, and Chinese coast guard vessels and fishing boats began plying waters around the islands.