China on Saturday bolstered its claim to islands that Japan says it owns, warning that it would take "defensive emergency measures" against aircraft that failed to identify themselves in airspace over them.
Ties between the Asian powers have been strained for months by the dispute over the East China Sea islands, called the Diaoyu in China and the Senkaku in Japan, which are believed to be surrounded by energy-rich waters.
China's government-run Xinhua news agency published a map and coordinates for the newly established "East China Sea Air Defence Identification Zone", which covers most of that sea including the disputed islands.
It also released Defence Ministry identification rules for aircraft in the area.
"China's armed forces will adopt defensive emergency measures to respond to aircraft that do not cooperate in the identification or refuse to follow the instructions," Xinhua said, adding the rules came into force on Saturday.
A Japanese Foreign Ministry spokesman said Japan lodged a severe protest at China's embassy in Tokyo and reiterated its position that the islands belong to Japan and China's action was not acceptable.
"Setting up such airspace unilaterally escalates the situations surrounding Senkaku islands and has danger of leading to and unexpected situation," Japan's Foreign Ministry said in a statement.
Patrol ships from both countries have been shadowing each other near the islets on and off for months, raising fears that a confrontation could develop into a clash.
There have also been several incidents involving military aircraft flying close to each other. In October, Chinese military aircraft flew near Japan three days in a row, and Japan scrambled fighter jets each time in response.
The new Chinese rules mean aircraft have to report flight plans to China's Foreign Ministry or civil aviation administration, maintain radio contact and reply promptly to identification inquiries, keep radar transponders turned on, and bear clear markings of their nationality and registration.
The Defence Ministry said it was the "administrative organ" for the zone, Xinhua said.
The zone is outside China's territorial airspace, but its establishment had a sound legal basis and accords with common international practices, Chinese Defense Ministry spokesman Yang Yujun said in an interview carried by Xinhua.
He noted that other countries had established similar zones and said China would put more in place in future.
"It is a necessary measure in China's exercise of self-defense rights. It has no particular target and will not affect the freedom of flight in relevant airspace," Yang said.