Japan says a Chinese military intelligence-gathering aircraft entered its territorial airspace off remote islands in the East China Sea on Monday, the first time Tokyo has accused the People’s Liberation Army Air Force of an airspace violation and a new irritant in frosty relations between China and Japan.

A map released by the Japanese Defense Ministry showed the Chinese aircraft, a Y-9 reconnaissance plane, flying in a rectangular circuit pattern off the eastern side of the Danjo Islands when it briefly headed west and crossed into the islands’ territorial airspace – which extends 12 nautical miles from the coast of the islands – for about two minutes.

The Japanese Defense Ministry said it scrambled Air Self-Defense Force fighter jets to respond to the alleged intrusion, but no confrontation with the Chinese plane was reported.

Japan’s Foreign Ministry said it summoned Shi Yong, charge d’affairs of the Chinese Embassy in Tokyo, “to protest extremely severely and strongly request the prevention of a recurrence.”

The Japanese military frequently scrambles fighter jets to confront Chinese military planes that approach but have not previously entered its territory, doing so 479 times – more than once a day – in the past fiscal year that ended in April, according to the Defense Ministry.

But the alleged incursion into sovereign airspace brings that tension to a higher level.

“In recent years, China’s military activities in the vicinity of our country have tended to expand and become more and more active,” Japanese Chief Cabinet Secretary Yoshimasa Hayashi said at a press conference Tuesday.

“The invasion of our country’s airspace by Chinese military aircraft is not only a serious violation of our country’s sovereignty, but also a threat to our safety, and is completely unacceptable,” Hayashi said.

The uninhabited Danjo Islands, a Japanese national monument and wildlife protection area, are in the East China Sea about 100 miles (160 kilometers) to the southwest of Nagasaki on the southern Japanese main island of Kyushu.

While this was the first reported incidence of a Chinese People’s Liberation Army Air Force aircraft violating Japanese airspace, there have been two similar incidents in the Japanese-controlled Senkaku Islands, which China calls the Diaoyus and claims as its sovereign territory, in the East China Sea.

In 2012, a Chinese maritime surveillance plane entered airspace around the Senkakus and in 2017 a drone launched from a China Coast Guard vessel did the same, according to the Japanese government.

But this week’s incident is the first time such an incursion has been attributed to a military plane.

The uninhabited Senkaku chain has been a sore spot in Japan-China relations for years.

Claims over the rocky islands, 1,200 miles (1,900 kilometers) southwest of Tokyo but only about 205 miles (330 kilometers) from China’s east coast, date back centuries, and neither Japan nor China is likely to back down over territory considered a national birthright in both capitals.

Tensions heated up in 2012, after Tokyo bought some of the islands from a private Japanese owner, which Beijing took as a direct challenge to its sovereignty claims.

It has frequently dispatched China Coast Guard and other government vessels to the waters around the islands to assert those claims, including keeping a Chinese presence near the islands for a record 158 straight days earlier this year, according to the Japanese government.

Any Japanese-Chinese incident in the Senkakus raises the risk of a wider conflict, analysts note, due to Japan’s mutual defense treaty with the United States.

Washington has made clear on numerous occasions that it considers the Senkakus to be covered by the pact.