China expressed “regret” Monday over South Korea’s declaration of an air defense zone overlapping Beijing’s in the latest ratcheting up of tensions over territorial disputes involving Japan, China and South Korea that the U.S. has been seeking to tamp down.

China urged Seoul to proceed “safely and cautiously” in dealing with the overlap from “South Korea’s expansion of its air defense identification zone,” said Chinese Foreign Ministry spokesman Hong Lei.

South Korea caught Beijing and Tokyo off guard Sunday with the announcement that Seoul’s existing air defense zone was being expanded about 150 miles to the south to include a submerged reef called Ieodo in South Korea and Suyan Rock in China.

Maj. Gen. Chang Hyok, a senior South Korean Defense Ministry official, said that in declaring the zone “our top priority is to prevent accidental military clashes in the area.”

The expansion meant that the air zones declared by South Korea, Japan and China over the East China Sea now all overlap. The Japanese and Chinese zones both include space over disputed islets called the Senkaku by Japan and Diaoyu by China.

Unlike the air defense zone announced by China two weeks ago, the South Korean zone will not affect civilian flights.

China angered its neighbors in declaring its own zone on Nov. 23 by ordering all military and commercial flights entering the zone to file flight plans and identify themselves or face possible “emergency measures.”

The U.S. immediately signaled that it would not recognize the Chinese zone by flying two B-52 bombers based in Guam through the Chinese zone without giving notification.

The air zone disputes dominated Vice President Joe Biden’s talks last week in Japan, China and South Korea.

“China’s recent and sudden announcement of the establishment of a new air defense identification zone has, to state the obvious, caused significant apprehension in the region,” Biden told a meeting of U.S. business executives in Beijing.

“The United States has a profound stake in what happens here because we need, and we are, and we will remain a Pacific power diplomatically, economically and militarily,” Biden said.