China has sent several fighter jets and an early warning aircraft into its new air defence zone over the East China Sea.
Japan and South Korea flew military aircraft through the zone yesterday, while the US sent two unarmed B-52 bombers into the airspace.
None of those aircraft informed the Chinese authorities.
China last week announced that foreign aircraft passing through its new air defence zone, including passenger planes, would have to identify themselves.
The zone includes the skies over islands at the heart of a territorial dispute between Japan and China.
The Chinese patrol mission was "a defensive measure and in line with international common practices", state news agency Xinhua cited an air force spokesman as saying.
The aircraft, including Russian-designed Su-30 fighter jets, conducted routine patrols and monitored targets in the zone.
However, Defence Ministry spokesman Yang Yujun said it was "incorrect" to suggest China would shoot down aircraft that entered the zone without first identifying themselves. He did not elaborate.
Japan's Chief Cabinet Secretary Yoshihide Suga said he did not know if Chinese planes were in the zone, but said there was no change to Japan's sense of alertness.
Ties between China and Japan have been strained for months by the dispute over the islands in the East China Sea, called the Diaoyu by China and the Senkaku by Japan.
The US does not take a position on the sovereignty of the islands, but recognises Japan’s administrative control and says the US-Japan security pact applies to them.
China's Foreign Ministry said yesterday that since the zone came into force there had been no impact on the safe operation of international civilian flights, although it added that China "hoped" airlines would cooperate.
Japan's two biggest airlines have defied the identification order since Wednesday at the request of the Japanese government.