A Chinese intelligence ship entered Japan's territorial waters early Wednesday morning, the first report of a Chinese navy vessel doing so in more than a decade.

A Japanese surveillance plane spotted the ship in waters west of Kuchinoerabu island before dawn, Deputy Chief Cabinet Secretary Hiroshige Seko told reporters. It sailed out of Japan's waters about 90 minutes later, he said.

A U.N. agreement allows what is known as "innocent passage" by foreign ships through another country's waters, but Japan nonetheless expressed concern to China that this incident and other recent Chinese military activity is escalating tensions between the two countries.

China's Defense Ministry said in a brief statement that the ship sailed through "a territorial strait open to international navigation" in accordance with freedom of navigation under the U.N. Convention on the Law of the Sea.

Japan reacted more harshly last week after a Chinese navy ship sailed near a group of remote islands that are administered by Japan but also claimed by China. Though the ship didn't enter territorial waters around the islands, known as the Senkaku in Japanese and the Diaoyu in Chinese, the dispute makes the situation there much more charged.

The Chinese Defense Ministry responded that the islands are China's inherent territory, and "it is rational and legal for Chinese warships to sail in waters under (Chinese) jurisdiction."

The latest incident took place much closer to Japan's main islands, in waters south of Kyushu. The only previous time Japan had reported a Chinese naval vessel in its territorial waters was a submarine detected in 2004.