“China requires the Japanese side stop illegal activities in the waters and airspace of the Diaoyu Islands,” Mr. Hong said.

The episode comes just ahead of Japanese elections in which conservatives pushing for a more robust military to counter China’s rise are in the lead.

For months, patrol ships from the two countries have sporadically faced off near the islands known as Senkaku in Japan and Diaoyu in China, exchanging protests over loudspeakers that each is infringing on the other’s sovereignty. Recently, Chinese ships have sailed near the islands more regularly in what analysts in Japan interpret as a new strategy by China, either trying to wear down Japan’s resolve or to use the patrols to bolster its claims that it is protecting the islands and therefore is in charge.

It was unclear on Thursday whether the plane’s flight might have been part of such a strategy. This week, China appeared to increase the pressure on Japan by sending a flotilla of navy ships near the islands, instead of the maritime surveillance ships it sent before.

Some analysts have said they consider the standoff more dangerous than separate conflicts Beijing is involved with over islands in the South China Sea because those pit China against countries less powerful than Japan, which has one of the world’s most sophisticated navies.