On Thursday, the top Japanese government spokesman, Chief Cabinet Secretary Yoshihide Suga, said that Japan had followed suit by sending an unspecified number of patrol planes into the airspace, though he did not specify exactly when they had flown. The aircraft patrolled the airspace on routine reconnaissance flights without incident, and China did not scramble its fighter jets to intercept them, Mr. Suga said.

Mr. Suga said that the aircraft had flown without informing China, defying Beijing’s demands that all traffic entering the so-called air defense identification zone file flight plans with China first. Japan and the United States have both refused to recognize the air zone, which covers the disputed islands, known as the Senkaku in Japan and Diaoyu in Chinese. The islands are administered by Japan, but also claimed by China.

The South Korean government also said that it had flown surveillance aircraft through the zone on Wednesday without alerting Beijing, a flight that Chinese officials said that they had monitored. Like Japan, South Korea claims sovereignty over territory in the zone, but enjoys warmer ties with Beijing than Japan does.