HONG KONG North Korea's claim to have tested a miniaturised hydrogen bomb, which would represent a big advance in its strike capability, could lead to renewed calls for countries in the region to enhance their missile defences, changes that China opposes.
Analysts and military attaches say Beijing will be watching closely for signs that the United States and Japan are exploring improved ballistic missile defence, or whether Washington leans more heavily on South Korea to adopt its advanced high-altitude missile system, technology Seoul has so far turned down.
There is considerable doubt over the veracity of Pyongyang's assertion that Wednesday's explosion was a full-fledged test of a hydrogen device, but it could mark an advance in the secretive state's nuclear technology.
It follows warnings from senior U.S. military officials last April that they believed North Korea had the ability to miniaturise a warhead and mount it on a ballistic missile, posing a potential new threat to the United States, Japan and South Korea.
Any moves to boost missile defences could inflame growing military rivalry between China and Washington and its allies.
That rivalry is playing out most visibly in the waters of the South China Sea, a vital trade route the United States fears Beijing wants to militarise, despite China's denials.
"China will be very sensitive about any moves by Japan or South Korea to improve missile defences," said Zhang Baohui, a mainland nuclear security expert at Hong Kong's Lingnan University.
"North Korea may be the stated reason, but Chinese strategists will it see as a move against China to limit its nuclear deterrence."
Zhang said he believed that, publicly at least, China would offer only a muted response to the latest test, given Beijing's desire for normalised ties with the isolated buffer state.
Richard Bitzinger, a security analyst at Singapore's S. Rajaratnam School of International Studies, said North Korea's actions potentially provided a "fig leaf" rationale for Japan to buttress its defences against its bigger security worry, China.
SOUTH KOREA AND THAAD
Chinese fears are particularly acute over sophisticated high-altitude ballistic missile defences that U.S. military officials have said were needed in South Korea, integrated with its own less-advanced systems.
The U.S. system involves batteries of missiles that could shoot down North Korea's as yet untested longest range ballistic missiles, weapons U.S. military strategists are most worried about given their potential to carry a nuclear warhead to the American west coast.
Acknowledging the concerns of its largest trade partner, China, South Korea has so far kept its missile defences independent of U.S. forces, and has avoided formal talks over the introduction of the so-called Terminal High Altitude Area Defense (THAAD) system.
The system has radars that can track multiple ballistic missiles up to 2,000 km (1,200 miles) away, a range which would reach deep into China.
Japan is also eyeing a future THAAD system, as well as a land-based version of the advanced Aegis system it uses at sea integrated with U.S. ships.
Industry sources said the perceived long-term threat from China and possible hyper-velocity missiles, rather than North Korea, was driving consideration of THAAD in Japan.
Japan's new security legislation passed last year and an earlier revision to security guidelines between Tokyo and Washington were already set to result in closer integration between the countries in ballistic missile defence capability on board Aegis-equipped ships in the Sea of Japan.
Japan is further looking to bolster its defence capability by fielding new interceptor missiles developed jointly with the United States.
Jeffrey Lewis, director of the East Asia Non-Proliferation Program at the Middlebury Institute of International Studies at Monterey, California, said North Korea has paraded two versions of a ballistic missile that appeared to be of a type with a range sufficient to hit the U.S. west coast, but has never tested it.
(Additional reporting by Tim Kelly in TOKYO and David Brunnstrom in WASHINGTON; Editing by Mike Collett-White)
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