Nearly a year after the US stepped into a simmering dispute between China and smaller countries in the region over potentially oil-rich islands in the South China Sea, tensions are rising again.

Since March, both Vietnam and the Philippines have accused Chinese forces of aggressive acts in disputed areas. Military experts say China’s sustained military buildup enables it to project more naval power in an oceanic region where the US Navy has long held sway. US Defense Secretary Robert Gates was due to meet Friday with his Chinese counterpart at a security summit in Singapore. “We are not trying to hold China down,” he told reporters Thursday.

Prior to the recent tension, however, analysts say China had begun to dial down its behavior and renew diplomatic efforts to win over its neighbors. The Chinese charm offensive began soon after Secretary of State Hillary Clinton used a regional security forum in Vietnam last July to stake out the US strategic interest in the South China Sea and offer to mediate peace talks.

While this offer hasn’t been taken up, the US intervention prompted Beijing to “recalibrate” its stance, says Susan Shirk, a former US diplomat and China expert who runs the Institute on Global Conflict and Cooperation at the University of California in San Diego. As a rising military power, China wants to avoid a pro-US tilt in the region. “This recalibration is about trying to get back to a more pragmatic and cooperative approach that China has, by and large, pursued since the 1990s,” she says.

A regional flashpoint

But regional governments have made scant progress on resolving overlapping claims on two island chains. The Association of Southeast Asian Nations (ASEAN) signed a broad code of conduct with China in 2002 as a way to calm tensions. But subsequent talks to agree on the rules have broken down amid criticism that ASEAN is too divided to act. Asian diplomats say China has tried to pick off weaker countries and head off a firm joint position on the South China Sea. Last year, Chinese Foreign Minister Yang Jiechi said “internationalizing” the issue would only make it harder to solve.

Indonesian Foreign Minister Marty Natalegawa said that Indonesia, the current chair of ASEAN, wanted to firm up the code of conduct by year-end on an issue that posed a “real security threat” to the region. He admitted that further delays would be a sign of failure. “Ten years with a lack of [implementing] guidelines is a bit much,” he told a security conference in Malaysia.

Most experts say that the complexity of the dispute, which involves six countries (China, Taiwan, Philippines, Vietnam, Brunei and Malaysia), scores of uninhabited islands and untapped hydrocarbons, resists any quick solution. Instead, it is likely to linger as a potential flashpoint, while countries continue probing the seabed to find out how much oil and gas could be recoverable. Estimates of their size and value vary widely.

On Sunday, Vietnam said that a Chinese patrol boat had deliberately cut the undersea cable of a Petrovietnam ship operating 120 nautical miles off the Vietnamese coast. Both countries claim that these waters lie inside their designated exclusive zones.

Dang Dinh Quy, the president of Vietnam’s diplomatic academy in Hanoi, says China has interfered with previous survey ships but that Vietnam took a quieter approach. He credits the US declaration of its strategic interest in the South China for Vietnam’s public assertiveness. “It’s made the position of the US clearer … and made everything more transparent,” he says.

Is it more about the US-China rivalry?

Not everyone welcomes the US intervention in the South China Sea. Hashim Djalal, a former Indonesian envoy on maritime diplomacy, who has chaired informal talks with representatives from China and other island claimants, is leery of a superpower rivalry in ASEAN. “America is looking at it as a China threat. That’s confrontational,” he says.

US officials have stressed their neutrality in territorial claims in the South China Sea. But analysts say it is hard to disentangle the tensions over island sovereignty from US-China military rivalry. Last year’s policy speech in Vietnam by Ms. Clinton came just months after a confrontation between Chinese patrol boats and a US naval surveillance ship operating near a Chinese submarine base on its southernmost island, Hainan. China said the US ship was spying inside its exclusive zone.

In the past, the Philippines and Vietnam have carried out joint oil exploration with Chinese ships in the vicinity of disputed islands. But these joint missions haven’t led to any agreement on how to tap oil and gas reserves since there is no consensus on how to proceed. “For China, there’s no reason to move to joint development [of oil fields]. It can afford to play a long game,” says Euan Graham, an expert of maritime security at Nanyang Technological University in Singapore.