Chinese experts have warned Japan not to play up the threat of China to escalate regional tensions, following a Chinese navy vessel's entrance into Japanese territorial waters, which experts said was an "innocent passage."

"The Tokara Strait is a territorial strait used for international navigation. It is in accordance with the freedom of navigation principle of the United Nations Convention on the Law of the Sea for Chinese warships to sail through the strait," China's Ministry of National Defense said on Wednesday.

Reports on the ship's entry splashed across Japanese media on Wednesday. Japan's Deputy Chief Cabinet Secretary Hiroshige Seko told a Wednesday news conference that the government will continue to take all possible warning and surveillance activities in their territorial waters and airspace, Kyodo News reported.

The report said that Japanese P-3C patrol aircraft spotted the Dongdiao-class intelligence vessel sailing in Japanese territorial waters to the west of Kuchinoerabu Island at around 3:30 am. The ship took a southeasterly route and exited Japanese territorial waters around 5 am.

A Japanese official was quoted by the Asahi Shimbun as saying that it is extremely rare to see a Chinese naval vessel in Japanese territorial waters.

The Yomiuri Shimbun also called it a second "invasion" of China's nuclear-powered submarine in Japanese territorial waters since 2004.

"According to the UN convention, any country's ships could pass the strait freely. It is clear that Japan is trying to play up the threat of China to cause regional chaos and further tamper with the South China Sea disputes," Lü Yaodong, director of the Institute of Japanese Studies at the Chinese Academy of Social Sciences, told the Global Times.

Lü added that Japan is trying to make the world believe that China is changing the status quo with forces. The country's ultimate goal is to contain China, he said.

Last week, Japan summoned the Chinese ambassador to express concern after a Chinese and three Russian warships had entered what Japan called the "contiguous zones" near the Diaoyu Islands almost simultaneously, Kyodo News reported.

Calling the navigation "legal and reasonable," China's defense ministry said that "China's naval ships have every right to navigate in waters under its jurisdiction, and no other country has the right to make thoughtless remarks about this."