The U.S. says it flew two military aircraft over a disputed area of the East China Sea without notifying China, in the first direct challenge to Beijing's bid to create an air defense notification zone.
Officials at the Pentagon Tuesday said B-52 bombers took part in regular exercises over disputed islands on Monday. There was no immediate reaction from Beijing.
The move follows earlier U.S. announcements that its military aircraft would not identify themselves according to the new Chinese rules.
The U.S. and Japan have vowed not to recognize the air defense identification zone, under which Beijing wants all civilian and military aircraft to identify themselves and obey its orders.
Japanese Defense Minister Itsunori Onodera said Tuesday that Tokyo is working closely with Washington and will take all necessary steps to protect Japanese territory.
"The defense ministry and self defense forces will look into the situation and do whatever it takes to protect our territory and airspace. As such, we will continue to look into what is needed to continue to monitor and patrol the area under self-defense and international law."
While U.S. and Japanese officials have condemned the Chinese move as provocative, they stressed it will have no effect on how they work in the area.
But the new regulations are affecting Japanese commercial flights. Several Japanese airliners say they are now notifying China when their planes enter the new zone.
Although some see this as an acknowledgement of Beijing's control of the area, the RAND Corporation's Scott Harold tells VOA this is not the primary intention of the new policy.
"The real goal of the air defense identification zone is not to try to deal with civilian airlines, since at all times those are expected to be open to announcing themselves and their intentions. It's really about trying to exclude Japanese or U.S. or Korean or even Taiwanese military airplanes from flying in those areas without China's permission."
Harold says the ultimate goal of the policy is to try to force the Japanese to the negotiating table by threatening to take action against any military plane that does not first seek Chinese permission.
The establishment of the new zone is the latest in a series of moves that have raised tension around the disputed East China Sea islands, known in Japan as Senkaku and in China as Diaoyu.
China has for months sent regular patrols of airplanes and ships near the islands, in what is seen as an attempt to challenge Japan's de facto control of them. The moves have raised concerns of an accidental clash.
Security analyst Yoichiro Sato of Japan's Ritsumeikan Asia-Pacific University tells VOA the moves are making it necessary for Washington to go further than its current level of support for Japan regarding the islands.
"This kind of strong move came in the wake of perceived weak U.S. commitment to the alliance and the defense of the Senkaku. This is a very dangerous move China is playing. And at this point, the U.S. has no choice but to upgrade its commitment. It has to. Otherwise, it will really cause a major problem later on for the United States."
The United States recognizes Japan's control of the East China Sea islands but says it takes no position on their "ultimate" sovereignty. It has, however, said the islands fall within the scope of the mutual defense treaty under which the United States is obligated to help Japan if attacked.