China on Saturday announced that it has demarcated an "air-defence identification zone" over an area of the East China Sea, covering islands that are also claimed by Japan.
The Chinese 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 China and Japan; and the latter country was prompt to lodge a protest over what it said was an "escalation".
The Chinese defence ministry has posted a map on its website detailing the zone's coordinates and rules governing the area, along with a question-and-answer statement from Yang Yajun, a ministry spokesman.
The move risks heightening tensions between Asia's two biggest economies, which are in dispute over the ownership of the islets, known as Senkaku in Japanese. The islands lie inside the zone and both nations claim sovereignty over the surrounding waters, which are rich in oil, natural gas and fish.
''This is a necessary measure taken by China in exercising its self-defence right,'' Yang said, according to the English-language statement. ''It is not directed against any specific country or target. It does not affect the freedom of over-flight in the related airspace.''
Junichi Ihara, the director-general for Asian and Oceanian affairs in the Japanese foreign ministry, protested to the minister at China's embassy in Tokyo by phone, the ministry said in an e-mailed statement.
Ihara told the minister Japan ''absolutely cannot accept'' the establishment of the zone covering the Senkaku islands, which he said ''are Japanese territory.''
The confrontation between China and Japan in the East China Sea has been building for more than a year and neither side shows any sign of backing down. China's declaration of an air-defence identification zone threatens to expand what has largely been a maritime contest into the air.
In recent weeks, China has flown bombers and a drone close to Japanese territory, prompting Japan to scramble fighter jets.
Beijing is now warning that it too could send up fighters if Japanese planes enter the disputed area. But both sides have been careful to avoid clashes in the area.
"Setting up such airspace unilaterally escalates the situations surrounding Senkaku islands and has danger of leading to an unexpected situation," Japan's foreign ministry said in a statement.
Taiwan, which also claims the Diaoyu/Senkaku islands, expressed regret at the move and promised that the military would take measure to protect national security.