The United States said it was "deeply concerned" and committed to defending Japan after China announced an air zone in the East China Sea that includes disputed islands.
In a move that US ally Japan branded as "very dangerous," China said it was setting up the "air defence identification zone" over the islands administered by Tokyo to "guard against potential air threats."
In similar statements, Secretary of State John Kerry and Secretary of Defence Chuck Hagel said yesterday that the United States was "deeply concerned" about the moves by China, which also scrambled air force jets to carry out a patrol mission in the newly declared zone.
"This unilateral action constitutes an attempt to change the status quo in the East China Sea," Kerry said.
"Escalatory action will only increase tensions in the region and create risks of an incident," the top US diplomat said from Geneva, where he was taking part in talks on reaching an agreement with Iran on its nuclear program.
Kerry said that the United States has urged China to "exercise caution and restraint," and warned Beijing against implementing its new zone.
"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.
Hagel reiterated that the Japanese-administered Senkaku islands - which the Chinese claim and call the Diaoyu - fell under the US-Japan security treaty, meaning that Washington would defend its ally Tokyo if the area is attacked.
"We are in close consultation with our allies and partners in the region, including Japan. We remain steadfast in our commitments to our allies and partners," Hagel said.
The defence chief made clear that the United States, which stations more than 70,000 troops in Japan and South Korea, would not respect China's declaration of control over the zone.
"This announcement by the People's Republic of China will not in any way change how the United States conducts military operations in the region," Hagel said.
The outline of the zone, which is shown on the Chinese defence ministry website and a state media Twitter account (pic.Twitter.Com/4a2vC6PH8O), covers a wide area of the East China Sea between South Korea and Taiwan that includes airspace above the disputed islands.
Japan last year nationalised the islands and has vowed not to cede sovereignty or even to acknowledge a dispute with China, accusing its growing neighbour of trying to change the status quo through intimidation.
China and Taiwan both claim the islands, which fall near potentially energy-rich waters.
