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.