China has demarcated an "air-defence identification zone" over an area of the East China Sea, covering islands that are also claimed by Japan.
China's defence ministry said aircraft entering the zone must obey its rules or face "emergency defensive measures".
The islands, known as Senkaku in Japan and Diaoyu in China, are a source of rising tension between the countries.
Japan lodged a strong protest over what it said was an "escalation".
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