A Chinese military surveillance plane breached Japanese airspace off the country’s southwestern coast on Monday, marking what Japan’s defense ministry described as the first known incursion by China’s military into its territorial airspace.

According to a ministry official, a Chinese reconnaissance aircraft briefly entered Japanese territory near Nagasaki Prefecture around 11:30 a.m. on Monday. In response, Japan’s Self-Defense Force put fighter jets on high alert and issued a warning to the Chinese aircraft.

While Chinese planes frequently appear in international airspace around Japan, this incident represents the first confirmed entry of a military aircraft into Japan’s territorial airspace.

Over the past two decades, Japan has increasingly faced foreign aircraft encroachments. Last year, Japan’s Self-Defense Force scrambled fighter jets to intercept foreign planes on 669 occasions — more than three times the number of such responses two decades ago.

Of these 669 cases, 479 were in response to Chinese aircraft sightings, according to Japan’s Ministry of Defense.

The incursion took place a day before Jake Sullivan, the White House national security adviser, is set to visit Beijing to hold talks with senior Chinese officials. The two sides are expected to discuss tense issues such as the status of Taiwan, the de facto independent island claimed by Beijing, and U.S. export controls of advanced technologies to China.

Military analysts suggest that Monday’s airspace violation could be a message from China challenging Japan’s delineation of its territorial border. China asserts control over a large continental shelf in the East China Sea, with its outer edge extending close to the Danjo Islands area, where the Chinese plane was spotted.

The airspace violation is the latest in a series of recent events heightening tensions between Japan and China.

Last week, a Chinese newscaster deviated from the script on a radio news program by Japan’s public broadcaster, asserting that the Senkaku Islands — controlled by Japan but claimed by China — are Chinese territory.

That same day, graffiti was discovered at the Yasukuni Shrine war-commemoration site in Tokyo using Chinese characters that appeared to read “toilet.”