A Japanese maritime self-defense force destroyer entered Chinese territorial waters recently in spite of warnings by Chinese vessels, Kyodo News reported Thursday.

The Suzutsuki temporarily ventured into waters off China’s eastern province of Zhejiang on July 4,  according to Kyodo. China’s Maritime Safety Administration, Zhejiang authorities had issued a no-sail zone from July 3 to July 5 where a live-fire drill was conducted in some waters in the East China Sea.

Japan’s defense ministry has started an investigation into the navy destroyer’s captain after Beijing raised “serious” concerns.

The Suzutsuki, which was sent to monitor the live-fire drill, was asked by Chinese vessels to leave the area when it advanced within 22 kilometers off the coast of Zhejiang. Instead, the vessel “sped up and navigated” into the Chinese waters for about 20 minutes before leaving.

The Chinese government considers the incident to be an act of “intentional provocation,” Kyodo News reported, citing sources.

The destroyer did not adhere to Chinese laws that foreign vessels need to seek prior authorization to enter, the report said, adding that Japanese destroyers in charge of surveilling Chinese vessels usually keep away from territorial waters off Zhejiang.

During unofficial talks between the parties, a Japanese official suggested that the entry could have been a “procedural error.” Japan is holding the position that the destroyer’s entry into Chinese waters was not illegal, Kyodo News reported.

The United Nations Convention on the Law of the Sea grants the right of innocent passage to traverse through territorial waters so long as the vessel does not compromise the safety of coastal countries. 

China often sends its coast guard and military vessels into Japanese territorial waters near the disputed Senkaku Islands in the East China Sea. The uninhabited islands are controlled by Japan, but China claims them as its territory.

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