Asian and Western diplomats said the zone was a problem for Japan, the United States and other countries that may be wary of any acknowledgement of China's claims over the area.
"No one wants to be in a position where by following Chinese instructions you are giving tacit acknowledgement of their sovereignty over a disputed area," one Asian diplomat said. "And there is a fear that is precisely the game that is being played - it seems no accident that the disputed Senkaku islands are now in the heart of overlapping zones."
Japan has its own Air Defence Identification Zone but officials said Tokyo only required aircraft seen to be approaching Japanese territorial airspace to identify themselves.
In its announcement on Saturday, China's Defence Ministry said it would set up other such zones when preparations were finalized. It gave no further details and the ministry's news department declined to elaborate when contacted by Reuters.
(Read more: Under Xi, China seeks to cool row with Japan over islands)
China also claims the South China Sea, which is believed to be rich in oil and gas. The Philippines, Vietnam, Malaysia, Brunei and Taiwan also claim parts of the South China Sea, making it one of the region's biggest flashpoints.
China patrols under way
China's official Xinhua news agency said the rules for the East China Sea came into effect on Saturday and that the Chinese air force conducted its first patrol over the zone. The patrol included early warning aircraft and fighters, it said.
Japanese Prime Minister Shinzo Abe said China was forcing other countries to conform to its rules.
Abe, who came to office last December promising to beef up the nation's military, has said Tokyo's door is open to dialogue to improve Sino-Japanese ties but has declined to acknowledge the existence of a formal territorial dispute over the islands, a step upon which Beijing insists.
"It's a unilateral step, changing the status quo in the East China Sea," Abe said in parliament on Monday.
China's Defence Ministry said it was within the country's right to set up the zone.
(Read more: Japan protests after Chinese plane flies over disputed isles)
"We reiterate that the purpose of China's approach is to defend national sovereignty and territorial airspace security, maintain the order of airspace flight, and is an effective exercise of our right of self-defence," spokesman Yang Yujun said in a statement.
Yang said China's move complied with international law.
"The United States, on the issue of the Diaoyu islands, must earnestly not take sides, not make inappropriate remarks and not give the wrong signal to Japan and encourage (its) risky behavior," Yang said.
Japan's own Air Defense Identification Zone extends around the Japanese archipelago and overlaps with China's new zone in part of the East China Sea.
"We might have more risk of encounters with Chinese aircraft in the area," said one former Japanese air force official. "We need to establish a system to avoid unnecessary incidents."
The topic was hot on China's Twitter-like Sina Weibo, with some users calling for war with Japan. "There can be no discussion on territorial issues, only war," wrote one user.