South Korea on Sunday said it had expanded its Air Defence Identification Zone (ADIZ) for the first time in six decades, in an apparent response to China’s November 23 decision to set up its first such zone over disputed parts of the East China Sea.

Underlining how Beijing’s decision has heightened regional tensions and, according to many analysts, increased the likelihood of an incident in the contested skies over northeastern Asia, South Korea’s newly expanded ADIZ will overlap with northern areas of China’s zone.

An ADIZ is not a territorial claim, but a defined area in international space within which countries monitor and track aircraft that are heading towards territorial airspace.

China’s November 23 announcement angered both Japan and South Korea, because the zone includes the disputed Diaoyu or Senkaku islands in the East China Sea, which are contested by Japan and China, and the submerged Leodo reef, which is under South Korea’s control. Both countries have made clear they would contest China’s zone by dispatching aircraft to carry out patrols through the area. China last week said it had scrambled jets to intercept at least 12 Japanese and U.S. aircraft.

Seoul’s announcement on Sunday was seen as an attempt to bolster its control over parts of the East China Sea surrounding the Leodo reef, which, earlier, lay beyond the southern edge of its zone. The eastern and western limits of its ADIZ have not been expanded.

The South Korean Defence Ministry in a statement said the new ADIZ was “designed to have its southern boundary match the country’s broader flight information region (FIR), and includes airspace over the South-controlled reef of Leodo and the islands of Marado and Hongdo,” the Seoul-based Yonhap news agency said. The government said the expanded zone would go into effect starting December 15, and would not impose restrictions on civilian flights.

The Defence Ministry said it had “offered sufficient explanations to related countries”. President Park Geun-hye is thought to have briefed U.S. Vice President Joe Biden on the move during talks on Friday. Mr.

Biden was in South Korea following stops in Tokyo and Beijing. His three-nation tour was aimed at boosting economic ties and reinforcing U.S. interests in the Pacific, although the visit was overshadowed by the tensions surrounding the ADIZ issue.

Mr. Biden was quoted as saying by Yonhap that he “appreciated President Park’s explanation and South Korea’s efforts”. An unnamed South Korean official added that he was of the view that South Korea’s ties with China, which had also been briefed about the move, would not “deteriorate seriously” because of the issue.

Chinese officials have defended their move by pointing to Japan’s already established - and larger - ADIZ in the East China Sea.

However, doubts about certain aspects regarding how China planned to enforce its control within the ADIZ raised anxieties. For instance, the Chinese government said it would take unspecified “emergency” defence measures if aircraft entered the zone without filing flight plans with Chinese authorities.

Even as Chinese officials have hit out at Tokyo for making “irresponsible remarks” about the issue, they have appeared to adopt a far more measured tone regarding concerns expressed by South Korea.

Asked how China would respond to a move by South Korea to expand its airspace, Foreign Ministry spokesperson Hong Lei told reporters on Friday China was “ready to stay in communication” with Seoul “based on the principle of equality and mutual respect”. He did, also, appear to rule out talks with Japan on the issue. “The point is,” he said, “Japan does not face up to history and reality, and fails to adopt the correct approach on relevant issues”.

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