Coming just three days before general elections in Japan, Thursday’s move sharply escalates territorial tensions that have already damaged economic relations between the two neighbors.
The Japanese government said eight F-15 fighter planes from the Air Self Defense Force were dispatched to airspace over the islands after coast-guard patrol vessels confirmed the presence of a single Chinese aircraft 15 kilometers, or about nine miles, south of the island Uotsuri Jima. That is the biggest of the contested chain, known as Senkaku in Japan and Diaoyu in China.
A defense ministry spokesman said this is the first time that a Chinese aircraft has intruded into any Japanese airspace.
The surprise incident underscores East Asia’s heightened security tensions in a year of important leadership changes for the region’s major players—China, Japan, South Korea and North Korea. It came only a day after North Korea launched a rocket that flew over the East China and South China seas in what is generally seen as a test of its ballistic-missile capacity. Earlier this week, Chinese warships passed through Japan’s contiguous waters near its westernmost island for the second time in about a month.
The island dispute remains an obstacle to relations between Japan and China, three months after Tokyo triggered the latest phase by buying three of the islands from a private owner.
The ties between Asia’s two largest economies could be strained further if Sunday’s election turns out as expected and opposition leader Shinzo Abe, known for his confrontational stance toward China, becomes prime minister.
Beijing has long contested Japan’s claim to the uninhabited islands, and in recent weeks has repeatedly sent patrol ships into the territorial waters around them, setting off cat-and-mouse chases with Japanese coast-guard vessels.
Both sides have prevented escalation by keeping their naval ships away, relying on civilian boats to patrol the area.
The Chinese plane sighted Thursday at 11:06 a.m. wasn’t a military aircraft. Warned by a Japanese patrol ship not to enter Japan’s airspace, it replied, “This is Chinese airspace,” Japan’s coast guard said.
Prime Minister Yoshihiko Noda said Tokyo has lodged a complaint with the Chinese government and will further step up surveillance of the area of dispute. “We will respond decisively to this violation of sovereignty,” he told reporters, according to Kyodo News.
In Beijing, foreign-ministry spokesman Hong Lei called the Chinese flight “entirely normal,” saying the territory the plane flew over rightfully belongs to China. He called on Japan to “stop illegal activities in China’s waters and airspace.”
The airspace incursion could heighten the dispute. International law forbids entering another nation’s airspace without permission and gives countries the right to expel immediately unauthorized aircraft with force. In contrast, foreign ships are able to sail through a nation’s territorial waters as long as it is considered “innocent passage.”
The incident also puts further pressure on Mr. Noda, whose ruling Democratic Party of Japan faces a decisive defeat in Sunday’s elections, according to various polls. Mr. Abe, who is likely to take his job away, has criticized Mr. Noda for his handling of the territorial issues.
The Chinese aircraft is a surveillance plane belonging to China’s State Oceanic Administration, according to Masayuki Masuda, a senior fellow at the National Institute for Defense Studies in Japan.
The plane is no match for a military jet, Mr. Masuda said.
—William Kazer in Beijing and Eleanor Warnock in Tokyo contributed to this article.
A version of this article appeared December 13, 2012, on page A11 in the U.S. edition of The Wall Street Journal, with the headline: Tokyo Responds to Chinese Plane, Sending Fighter Jets.