Randall Schriver, assistant secretary of defense for Asian and Pacific security affairs, gave the assurance as tensions rise over China's aggressive stance towards its claims to most of the strategic waterway.
Speaking to reporters at the US embassy in Manila, Schriver was asked if the United States, which is bound to the Philippines by a mutual defence treaty, would help the country if China invaded the main Filipino-occupied island in the South China Sea.
"We'll be a good ally... there should be no misunderstanding or lack of clarity on the spirit and the nature of our commitment," he said.
"We'll help the Philippines respond accordingly," Schriver added, declining to give further details.
China and the Philippines, along with several other Asian nations, have conflicting claims to parts of the South China Sea, which is believed to contain vast mineral resources.
China has built up heavily defended artificial islands in the sea and Philippine observers have expressed concern in recent weeks over aggressive Chinese radio warnings to foreign planes against approaching them.
However Schriver said US forces would continue to fly and sail in those waters.
"We've seen an increase in this kind of challenge from China, not only directed at us but others," he said.
"This kind of challenge will not result in a change of our behaviour. We'll not allow them to rewrite the rules of the road or change international law".
Since he took office in mid-2016, Philippine President Rodrigo Duterte has been moving the country's relations away from its traditional ally the United States and towards China and Russia.
Schriver also cautioned the Philippines against buying large-scale weapons like submarines from Russia as some local officials have suggested.
"I don't think that's a helpful thing to the (US-Philippine) alliance and ultimately I think we can be a better partner than the Russians," he said.
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