A Chinese Foreign Ministry spokesman, Hong Lei, refused to say how long China would send its ships into waters around the islands. However, he said the Chinese surveillance ships were “completely justified,” describing their maneuvers as “a normal part of their duty” in overseeing China’s sovereign interests.

China also appears to be ratcheting up pressure by broadening its effort to use current and former senior officials to make China’s case to the world. In a speech in Hong Kong this week, Chen Jian, China’s former ambassador to Japan, took aim at the United States, saying its support of its ally Japan was encouraging what he called a return of Japanese militarism.

Other Chinese officials and news media have taken a similar view, blaming a swing in Japan to the nationalist right for the tensions. However, analysts in Tokyo, and some in the United States, say China has seized upon Japan’s purchase of the islands as a pretext for pressing its longstanding claims.

They said that the continuing Chinese pressure has seemed to confirm a growing sense of Japanese insecurity over China’s rising economic and military presence in the region, as well as the relative decline of both itself and its longtime protector, the United States. That nervousness has made Japan more willing to push back. “It is disingenuous to claim the Japanese caused this problem,” said Kevin Maher, a former United States diplomat in Japan who is now a senior adviser at NMV Consulting, based in New York. “But this has been an eye-opening experience for people in Japan to see that the security environment in East Asia can be a dangerous place.”

Japan’s anxieties have also provided at least some opportunity for a small but vocal group of nationalists like Tokyo’s governor, Shintaro Ishihara, whom many in Japan blame for starting the current flare-up with China by proclaiming in the spring that he would buy the islands.

Analysts said it was unlikely that China would go as far as attempting a forceful takeover of the islands. Rather, said Mr. Maher, the goal may be to try to undermine Japan’s claims under international law that it wields effective control over them, while building a legal basis for making similar claims of its own.

China’s motives appear to be partly economic: the Chinese economy’s hunger for the petroleum and natural gas that scientists believe lie under the ocean floor surrounding the islands. Analysts also cited a Chinese desire for payback for Japan’s brutal World War II-era invasion of China. China says Japan seized the islands from China in 1895 as a first step toward Japan’s empire-building in Asia. Japan says it annexed islands that were not claimed by any nation.