TOKYO—Japan scrambled fighter jets over contested islands in the East China Sea after a small Chinese propeller plane entered what Tokyo considers its airspace.

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.