China has declared its first-ever air defence zone covering the East China Sea, in a move expected to escalate long simmering tensions with Japan, possibly even raising the chance of conflict on - and over - the high seas.
In response, Tokyo has lodged a "strong protest" over the zone's establishment.
The new zone overlaps with a similar zone that Japan has; it covers a wide area of the East China Sea between South Korea and Taiwan that includes the Tokyo-controlled islands known as Senkaku to Tokyo, but claimed and known as Diaoyu by Beijing.
Beijing also issued a set of rules for the zone, known as the East China Sea Air Defence Identification Zone, saying it will adopt "defensive emergency measures" on aircraft that defy its rules, although it did not elaborate further.
The Defence Ministry said on Saturday morning, as it announced the establishment of the zone which kicked in with immediate effect, that this was a "necessary measure" as China exercises its right to self-defence.
"It is not directed against any specific country or target," spokesman Yang Yujun added on the ministry's website.
China will establish similar zones "at the right moment after necessary preparations", he said.
Mr Junichi Ihara, who heads the Japanese Foreign Ministry's Asian and Oceanian affairs bureau, made a protest by phone to Mr Han Zhiqiang, minister at the Chinese Embassy in Japan, the ministry said in a statement. Japan could "never accept the zone" as it includes the Tokyo-controlled Senkaku islands, the statement said.
Experts say the zone is likely to sour Sino-Japan ties further even as a Japanese business delegation visited Beijing last week, hoping to mend ties between the two sides in the light of the tense political climate.
But the Chinese position could be a reaction to recent talk of Tokyo's plans to shoot down drones that invade Japanese airspace, they said, and an attempt to prevent its military activities near the disputed islands from being blocked.