BEIJING - China's efforts to protect its submarine gateway to the South China Sea could broaden from stand-offs with US military planes to announcing an air defence identification zone (ADIZ), according to two retired army officers.

China could seek to restrict the air space around Hainan Island as the US routinely runs surveillance flights in the area, retired colonel Yue Gang said. The move would mirror a zone China set up in November over portions of the East China Sea disputed with Japan.

"Although it's premature to set up an air zone over the entire South China Sea at this moment, it makes sense to build a partial zone covering the waters near Hainan, where China's biggest nuclear submarine base is located," Mr Yue said. Encompassing air space deemed international by the US and by China as being within its exclusive economic zone, it would draw "red lines" for US military flights.

The Chinese foreign ministry has previously said that China has the right to adopt any security measures, including air zones, while calling reports of plans for one over the South China Sea "speculation".

Any move towards an ADIZ, which would follow the decision in January to require foreign fishing boats to seek permission before entering waters off Hainan, could raise the risk of confrontation between the air forces of China and the United States.

President Xi Jinping has sought to extend China's reach since coming to power in November 2012, with defence spending rising 12.2 per cent this year. The Communist Party leadership has for the first time stated a national goal of making China a maritime power, with a more combat-ready military and long-range capacity to bolster its claims to a large part of the South China Sea.

In an incident on Aug 19, a Chinese fighter jet flew within 6m of a US surveillance aircraft near Hainan, and rolled to expose its weapons to the pilot, Pentagon spokesman John Kirby said. The US plane was in international airspace and the manoeuvre was unsafe, he said. China said its pilot acted professionally.

Announcing an air zone across the entire South China Sea would be complicated as it would directly affect more countries in the area, according to retired major-general Xu Guangyu, a senior adviser at a Beijing-based research group, the China Arms Control and Disarmament Association.

Countries such as the Philippines, Vietnam, Malaysia and Brunei also claim part of the waters.

"Beijing will exercise caution in launching a South China Sea ADIZ," said Mr Xu.

"But it will be a matter of time. China's air defence space won't be just confined to the East China Sea."

BLOOMBERG