MANILA — President Benigno S. Aquino III said Thursday that the United States would be obligated to take military action in the South China Sea if China moved to reclaim a hotly contested reef directly off the Philippine shore.
Mr. Aquino said he had no indication that China had imminent plans to develop the reef, known as Scarborough Shoal, which sits 185 nautical miles from Manila.
But he suggested that there would be a harsh response if China decided to do so, saying that in his view the United States would be forced to defend the Philippines or risk losing its credibility in the region.
“It has to maintain its ascendancy, moral ascendancy, and also the confidence of one of its allies,” Mr. Aquino said during an interview at the presidential palace in Manila.
China has moved swiftly in recent years to strengthen its presence in the South China Sea, one of the world’s major shipping routes, by building artificial islands equipped with airstrips and radar on top of rocks and shoals. Its actions have angered neighboring countries, including the Philippines, which claim many of the territories as their own.
If China were to succeed in making Scarborough Shoal a strategic outpost, it would be a major coup, given its proximity to American and Philippine military forces. The United States recently gained approval from the Philippines to begin stationing troops at five bases, in hopes of deterring China.
Mr. Aquino said he had not seen any recent intelligence reflecting a Chinese buildup at the shoal, which China effectively took control of in 2012, after a long standoff. But he said the Philippines would be prepared for any action by China. “We don’t subscribe to the notion that it’s theirs,” he said.
In recent weeks, Chinese military commentators have indicated that Beijing would like to begin developing the shoal, possibly by adding an airstrip. A recent report by Xinhua, the official news agency, called Scarborough Shoal an “inalienable part of the Chinese territory” since ancient times.
Military analysts have suggested that efforts by China to reclaim the shoal could prompt an aggressive response from the United States.
The United States and the Philippines have a mutual defense treaty, though experts said it would probably not be applied unless the Chinese military attacked Philippine forces unprovoked.
“Scarborough is a red line,” said Gregory B. Poling, director of the Asia Maritime Transparency Initiative at the Center for Strategic and International Studies in Washington. “It would clearly change the balance of power.”
The Philippines has disputed China’s claims in the South China Sea before an international court at The Hague. A decision is expected in the coming weeks, though China has vowed to ignore it.
In the interview, Mr. Aquino, whose six-year term as president ends next month, compared the dispute in the South China Sea to a boxing match, saying China, with its vast population and considerable economic and military assets, was capable of knocking out the Philippines at any time.
“I think it would be foolhardy for us to devote all of our resources to getting weapons of mass destruction,” he said.
Mr. Aquino brushed aside concerns that his successor, Rodrigo Duterte, would take a softer approach toward China, saying the president-elect might come to see things his way once he had a full briefing.
Mr. Duterte has indicated that he would be open to negotiating directly with the Chinese rather than pursuing international methods to resolve the dispute.
At the same time, Mr. Duterte has made patriotic statements about the sovereignty of the Philippines; during the campaign, he said he would ride a Jet Ski to the disputed territories and plant a Philippine flag.
In assessing his mistakes as president, Mr. Aquino said he wished he had devoted more time to communicating his achievements.
He said he felt that he was sometimes held accountable for problems outside of his control. Some Filipinos, he said, still saw the president as all-powerful, a vestige of authoritarian times.
“When the ceiling of the airport collapses,” he said with a smile, “it’s my fault.”
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