"US officials will be under considerable pressure to clarify how, if it all, the US would seek to enforce its admonition about China forcing changes in the status quo," said Daniel Sneider, associate director for research at the Shorenstein Asia-Pacific Research Centre at Stanford University in California. "At what point, others may rightfully ask, do Chinese actions in the South China Sea constitute a threat to that order and what, if anything, would the US do about that?"

Vietnam's prime minister, Nguyen Tan Dung, has called for a "stronger voice" from the US against China after clashes between coastguard vessels near an oil rig that China placed in contested waters off Vietnam's coast. The Philippines has sought support from the US and the United Nations to counter China's encroachment into shoals off its coast.

The annual Shangri-La security dialogue in Singapore will bring together defence ministers and military leaders from around the globe. Japanese Prime Minister Shinzo Abe gave the keynote address last night, while China has sent Fu Ying , a former deputy foreign minister, alongside a retinue of People's Liberation Army officers.

When he visited East Asia last month, Obama affirmed that US treaty obligations with Japan covered islands in the East China Sea also claimed by China and said US commitments were "ironclad". But in a speech on Wednesday during a graduation ceremony at the US Military Academy in West Point, New York, Obama said not every problem "has a military solution".

"Since World War II, some of our most costly mistakes came not from our restraint, but from our willingness to rush into military adventures," he said.

"President Obama's West Point speech will not ease misgivings among Asian partners about the US commitment to the region's security," said Rory Medcalf, director of the International Security Programme at the Lowy Institute for International Policy in Sydney. "The speech sits uneasily with the idea of a rebalance to Asia. It sends out mixed signals to Asian countries about what really constitutes an American core interest in this region."

The administration faces distractions in the Middle East and from the Ukraine crisis. The exit in stages of the US Army from Afghanistan after the longest war in US history, along with budget cuts and the failure to wrap up a US-led Pacific trade pact have further raised doubts about its Asia focus at a time when China is pressing its agenda in the region.

During a visit to Beijing last month, Hagel was told by his counterpart, General Chang Wanquan , that China would make "no compromise, no concessions" in disputes with Japan and the Philippines. "The Chinese military can assemble as soon as summoned, fight any battle and win," Chang said. China could not be contained, Chang added, and the Pacific was "huge enough" to hold both countries.

"We urge the United States to take an objective and fair attitude, speak and act cautiously, and avoid contributing to several countries' provocation," Ministry of National Defence spokesman Geng Yansheng said at a briefing in Beijing yesterday.

The US has a huge interest in keeping sea lanes in the region open for commerce, Hagel told reporters on a military aircraft heading to Singapore. He plans to discuss the South China Sea tensions in "some specific terms" and saw areas where China was "overplaying its hand".

China's official defence spending, while less than a third of the US, is narrowing the gap. Its military budget will rise 12.2 per cent this year as Xi seeks a more combat-ready army and a navy with broader reach throughout the Pacific.

The US has 38,000 troops stationed in Japan, with about half on Okinawa, and 28,500 in South Korea. There are about 80,000 US soldiers in the Pacific region, according to the US Army Pacific's website, and Secretary of State John Kerry said in January the US would send a further 800 troops to South Korea with upgraded equipment.

The US has urged the Association of Southeast Asian Nations and China to agree on a code of conduct for the South China Sea and called China's actions "provocative" after the clashes with Vietnam. Even nations that have kept out of the territorial disputes are now being drawn in.

Bonnie Glaser, senior adviser for Asia at the Centre for Strategic and International Studies in Washington, expects Hagel, during his trip to Singapore, to review some of the specific policies and actions of the US. She also said: "In recent years the US secretary of defence has not criticised China harshly at the Shangri-La Dialogue, but this year could be different."

The US could deploy more troops and equipment to the region, such as extra aircraft carriers, according to Richard Bitzinger, a senior fellow at the S. Rajaratnam School of International Studies in Singapore. Still, "China would have to really do something heinous before the United States would actually take military action," he said.

"I would say the United States would be trying more likely to play nice, to talk about common security and stuff like that" at the Singapore meeting, Btizinger said.

China has sent its defence minister only once to the summit, in 2011, according to Glaser. "Usually they send a deputy chief of the PLA general staff," she said. "Sending Fu Ying is a departure from that practice. She has a reputation for being a tough 'iron lady'."

Lieutenant General Wang Guanzhong , the deputy chief of general staff of the PLA, is leading China's military delegation, according to the International Institute for Strategic Studies, the meeting's organiser.

Sending Fu, chairwoman of the Foreign Affairs Committee of the National People's Congress, could indicate China expects a hostile audience in Japan. Abe told parliament in Tokoy this week he planned to make his case at the summit for Japan broadening the scope of its military. Hagel and Abe are scheduled to meet, and officials from the US, South Korea and Japan will also meet.

Abe is seeking to increase Japan's influence in Asia by bolstering trade ties and providing patrol boats to countries such as the Philippines. On Wednesday. he called China's actions "extremely dangerous" after Chinese fighter jets flew unusually close to Japanese military planes near the disputed East China Sea islands.

With tensions high, "the US needs to reinforce President Obama's recent assurances to allies in Asia", said the Lowy Institute's Rory Medcalf. "But it will take more than another speech to manage the quiet misgivings in Asia about America's willingness to take risks in support of partners or even allies."