When Hillary Clinton said in July that the resolution of territorial disputes in the South China Sea was in the US national interest, she threatened to upset a delicate diplomatic balance that has been maintained for a decade.

That restraint has held in check a potentially explosive cocktail composed of China’s pursuit of energy security, its neighbours’ fears about its growing power and US concerns about Beijing’s military expansion in an oil and gas-rich area. The sea is also home to vital shipping lanes.

The US secretary of state drew attention to Washington’s anxiety about the region when she added her voice to calls for a multilateral approach to resolving territorial disputes in the South China Sea during an Asian security forum, calling the issue a “leading diplomatic priority”.

China, Vietnam, Malaysia, the Philippines, Brunei, Taiwan and Indonesia lay claims to all or part of the body of water that extends from China’s Hainan island to the northern coast of Borneo. The areas with the most overlapping claims are mainly near the Spratly and Paracel island groups.

In 2002, China and the Association of South East Asian Nations (Asean) signed a declaration of conduct, under which all parties pledged to exercise restraint in the disputed waters. But Asean members feel that the accord has become meaningless, and observers speak of Chinese assertiveness in the South China Sea, while disputes between fishing vessels and naval forces are on the rise. What has for many years been a dispute between neighbours is also gaining a global dimension.

A new submarine base on Hainan is expected to result in China’s navy becoming more active in regional waters. This could restrict the US navy’s freedom of movement in the area.

Beijing also recently called the South China Sea a “core interest” – a status it had in the past only assigned to Taiwan and Tibet, although Beijing made its reference “in the context of specific US activities that China opposed, not the dispute more generally”, said Taylor Fravel, an expert on Chinese security issues at Massachusetts Institute of Technology.

“The Chinese may see parts of the South China Sea as the potential redoubt for their future ballistic missile-carrying submarines,” said Tim Huxley of the International Institute for Strategic Studies.

“China’s greater interest in the South China Sea [is] not just about natural resources. There are hard strategic issues as well.”

When Beijing issued its first official reaction to Mrs Clinton’s comments, it made a point of condemning the “internationalisation” of the dispute and also disclosed that it had conducted a naval exercise in the area last week.

The view in Washington is that Mrs Clinton’s comments reflect US concerns that, left unchecked, China’s military drive could transform the region’s political landscape.

“China is shifting its military focus from a land-centric focus to an air and maritime-focused capability,” Admiral Mike Mullen, chairman of the US joint chiefs of staff, said in India recently. “The Pacific reg­ion is a critical economic region; a critical trade region . . . [and so] I’ve gone from being curious about where China’s headed to being concerned about it.”

South-east Asian governments have also expressed concern.

Indonesia is leading efforts to try to forge a regional consensus.

In a recent letter to the UN, Jakarta said China’s attempt to use isolated islets to establish its legal claim to sovereignty “clearly lacks international legal basis”, and to allow it would undermine the UN Convention on the Law of the Sea – the framework for establishing sovereignty.

“[China’s military expansion] is a fact,” said Teuku Faizasyah, a spokesman for Indonesia’s foreign ministry. China’s growth was increasing suspicion in the region, he said.

Vietnam, and to a lesser extent Malaysia, have also begun to push back against what some see as Chinese bullying, says Mr Huxley.

From China’s perspective, things look different. Li Jinming, a South China Sea expert at Xiamen University, says China is concerned about Vietnam pushing ahead with oil exploration in disputed areas.

A UN deadline early last year for coastal states to file claims for stretches of seabed linked to their continental shelf made things worse. Claims filed by the Philippines, Vietnam and Malaysia triggered an intervention from China, which claims almost all of the South China Sea.

A Philippine foreign ministry official said Asean members wanted a meeting of senior officials from the regional group and China to discuss the drafting of a binding code of conduct. “But China is non-committal at this time,” he added.

Beijing believes that time is on its side.

Song Xiaojun, a Beijing-based military analyst, says Asean countries are betting on the rivalry between China and the US, with “a foot on one boat each”. But he thinks China’s continued economic rise will change the odds eventually.

Additional reporting by Anthony Deutsch, Roel Landingin and Daniel Dombey

Get alerts on Emerging markets when a new story is published

Copyright The Financial Times Limited 2019. All rights reserved.

Comments have not been enabled for this article.

Follow the topics in this article