China Coast Guard vessels began last month to issue warnings for Japanese Self-Defense Forces planes to leave airspace over and around the Japan-controlled, China-claimed Senkaku Islands in the East China Sea, sources familiar with the matter said Saturday.

The warnings via radio communications are based on Beijing's claim that SDF aircraft could violate Chinese territorial airspace around the islands, called Diaoyu in China, and have been made several times since January, the sources said.

File photo taken in September 2013 shows the Japan-controlled Senkaku Islands in the East China Sea. (Kyodo)

They said Tokyo has refuted the claim and told Beijing through diplomatic channels that such warnings are "totally unacceptable," without publicly announcing the move in an apparent effort not to damage bilateral relations further.

The development came after Chinese President Xi Jinping called for bolstering Beijing's sovereignty claim over the islets during a rare visit in November to the command office for the East China Sea area of the China Coast Guard in Shanghai.

In the past, Chinese military vessels have urged Japanese Maritime Self-Defense Force aircraft to leave airspace near the Senkakus. A Japanese government official said it marks the first time for China Coast Guard ships that regularly sail around the isles to issue such warnings to SDF planes in Japanese territorial airspace.

The issuance of warnings may have been newly added to tasks of the China Coast Guard, which frequently sends ships to waters near the Senkakus and chases Japanese fishing boats in the sea area.

During the Nov. 29 visit, Xi pointed out the need for Beijing to "constantly strengthen" its efforts to safeguard the sovereignty of the Senkaku Islands, according to sources close to the matter.

The coast guard has subsequently drafted a plan to keep the presence of its ships near the islets every day in 2024 and conduct inspections of Japanese fishing boats in the sea area, if necessary.

In 2023, Chinese vessels were spotted near the Senkakus on 352 days, hitting a record-high figure since Japan brought the islets under state control in 2012.

Among China Coast Guard vessels that issued warnings to SDF aircraft while sailing in the contiguous zone outside Japanese territorial waters was a ship equipped with a 76-millimeter rapid-fire gun with a range of about a dozen kilometers, according to the sources.

The row over the Senkakus has been a longstanding source of tension between the two Asian neighbors. Since the escalation of the situation in 2012, Chinese coast guard vessels have repeatedly intruded into Japanese waters near the islands.

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