The United States has sharply criticised China's move to impose new rules on airspace over islands at the heart of a territorial dispute with Japan, calling it a "destabilising attempt to alter the status quo in the region".
China's government-run Xinhua news agency published coordinates for a newly established "East China Sea Air Defence Identification Zone," which covers most of that sea and includes the skies over the disputed islands.
China warned that it would take "defensive emergency measures" against aircraft that failed to identify themselves properly in the airspace.
On Saturday, the White House, Pentagon and State Department all voiced concerns about the move.
The US defence secretary, Chuck Hagel, in a strongly worded statement, said China's announcement would not change how the US carries out military operations in the region.
"We view this development as a destabilising attempt to alter the status quo in the region," Hagel said.
"This unilateral action increases the risk of misunderstanding and miscalculations."
Many countries including the US have air defence identification zones, which extend beyond a nation's sovereign airspace.
The US requires approaching foreign aircraft to follow its air defence identification zone procedures only if they intend to enter US national airspace.
But China's announcement suggests that foreign aircraft merely passing through that zone would have to follow China's procedures - or face unknown, potentially dangerous consequences.
One US official, speaking on condition of anonymity, said China's move was being interpreted as "a direct challenge" to Japan's operations in the area.
The US secretary of state John Kerry said freedom of overflight was essential to stability and security in the Pacific.
"We urge China not to implement its threat to take action against aircraft that do not identify themselves or obey orders from Beijing," Kerry said in a statement.
Tensions in the region are expected to be a topic of discussion when the US vice president, Joe Biden, travels to China, Japan and South Korea in early December.
The US has a hefty military presence in Japan, including on the southern island of Okinawa, which is close to the disputed isles.
"The concern would be, what happens if China tries to regulate aircraft that are overflying the Senkakus," a second US official told Reuters, using the Japanese name for the islands that China calls the Diaoyu.
The islands are believed to be surrounded by energy-rich seabed.
Xinhua said the rules came into force on Saturday and the Chinese air force conducted its first patrol over the zone.
The patrol included early warning aircraft and fighters, it said.
Japan scrambled fighter jets on Saturday afternoon against two Chinese reconnaissance planes over the East China Sea, the Japanese Defence Ministry said.