China has sent warplanes to its newly declared air defence zone in the East China Sea, state media reports.
The vast zone, announced last week, covers territory claimed by China, Japan, Taiwan and South Korea.
China has said all planes transiting the zone must file flight plans and identify themselves, or face "defensive emergency measures", the BBC reported.
But Japan, South Korea and the United States have all since flown military aircraft through the area.
The new dispute in an already tense region has raised concerns it could escalate into an unplanned military incident.
China's state news agency, Xinhua, quoted air force spokesman Col Shen Jinke as saying several fighter jets and an early warning aircraft had been deployed to carry out routine patrols as "a defensive measure and in line with international common practices".
He said the country's air force would remain on high alert and would take measures to deal with all air threats to protect national security.
In Xinhua's Chinese language version of the article, the colonel said the aircraft would "strengthen the monitoring of targets in the air defence zone and do their duty".
Last week, China announced a vast air defence perimeter covering territory claimed by China, Japan, Taiwan and South Korea. It said all planes transiting the zone must file flight plans and identify themselves, or face "defensive emergency measures".
The United States has since flown two B-52 bombers over the zone, and Japan and South Korea subsequently sent military aircraft through the area, the BBC reported.
The controversial air defence identification zone includes islands known as Senkaku in Japan and Diaoyu in China, which are claimed by Japan, China and Taiwan.
Japan controls the islands, which have been the focus of a bitter and long-running dispute between Japan and China.
The zone also covers a submerged rock that South Korea says forms part of its territory.