U.S. Defense Secretary Chuck Hagel has reaffirmed the U.S.-Japan defense treaty applies to a group of disputed islands that were included in China's bid to establish an air defense zone.
A Pentagon spokesman says Hagel spoke Wednesday with his Japanese counterpart, Itsunori Onodera, to discuss the security situation in the East China Sea.
The spokesman says the U.S. defense chief "commended the Japanese government for exercising appropriate restraint in the wake of [China's] announcement."
Senior Obama administration officials say U.S. Vice President Joe Biden will discuss the issue with officials in China next week as part of his upcoming three-nation tour of the region, which will include a stop in Japan.
On Tuesday, the Pentagon announced it had flown two unarmed B-52 bombers near the disputed islands in the first direct challenge to Beijing's bid to establish an air defense zone.
China's Defense Ministry said Wednesday it monitored "the entire course of the flights and identified them in a timely way." It warned "China is capable of exercising effective control" over the area.
The Pentagon said the Monday flights did not trigger an immediate response from Beijing, which two days earlier had declared the airspace part of a new air defense identification zone. China warned all aircraft to identify themselves before entering the area and obey all orders from Beijing.
U.S. officials describe the Monday flights as part of long-planned and routine training missions. But analysts say it was a clear message that Washington will not recognize China's attempt to establish control over the area.
Ralph Cossa of the Hawaii-based Pacific Forum tells VOA the B-52 flights were a "quick and appropriate" response to what is seen by many as a Chinese escalation.
"I think it was important to quickly demonstrate that we were not going to essentially allow the Chinese to start carving out international airspace that others cannot use."
China published coordinates for the so-called East China Sea Air Identification Zone on Saturday and warned it would take emergency defense measures to enforce its claim.
But it is not clear to what extent China will enforce the new rules.
Herman Finley, an associate professor at the Asia-Pacific Center, tells VOA that while it is not likely China will back down, it is probably not looking for a confrontation at this time.
"(The Chinese) push, they see what the reaction is, and then they push back when they see there's an opportunity. They've made their point. They'll wait and see when there's an appropriate, relatively predictable time to reassert their prerogatives."
Some analysts have described the Chinese move as a miscalculation, saying it may have underestimated U.S. resolve to protect the interests of its ally, Japan.
Michael McKinley with the Australia National University tells VOA that China is "pushing its luck" in this regard.
"It's attempting to see what is in the realm of the possible and the tolerable. The problem is that it's going to run into increasing resistance particularly from those who think it would be better now rather than later to confront China with some higher form of force."
The uninhabited islands -- known as Senkaku in Japan and Diaoyu in China -- were annexed by Japan in the late 19th century. China claimed sovereignty over the archipelago in 1971. Beijing linked its claims to ancient maps it says shows the territory has been Chinese for centuries.
The festering dispute is one of several maritime controversies pitting China against Southeast Asian nations, including the Philippines, Vietnam, Brunei and Malaysia.
Beijing has indicated a willingness to negotiate the disputes, but has so far rejected calls for multilateral talks. It has sought separate talks with each country.