MANILA -- China's island-building and military activities in the South China Sea "threaten" the Philippines' sovereignty and security, the United States' top diplomat said Friday, as he assured Manila that an "any armed attack" would prompt Washington to act under a 67-year-old Mutual Defense Treaty.

Beijing has build massive structures on reefs claimed by the Philippines in the waterway, where $5 trillion worth of trade passes annually. The construction proceeded despite Manila's win before a United Nations-backed arbitration court.

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"As an island nation, the Philippines depends on free and unobstructed access to the seas. China's island-building and military acts in the South China Sea threaten your sovereignty, security and therefore economic livelihood, as well as that of the US," US Secretary of State Mike Pompeo said in a joint press conference with Philippine Foreign Affairs Sec. Teodoro Locsin Jr.

"As the South China Sea is part of the Pacific, any armed attack on Philippine forces, aircraft or public vessels in the South China Sea will trigger mutual defense obligations under Article 4 of our Mutual Defense Treaty," Pompeo said.

Washington's top diplomat added, "Our commitments under the treaty are clear. Our obligations are real. The South China Sea is certainly part of an important body of water for freedom of navigation," he said.

Locsin said calls to review the Mutual Defense Treaty would require "further thought." He said there must be a "sincere desire" between the two parties "to help and be helped."

"My own view, it is a dynamic exchange that's going on in government, my own view is no... In the old theory of deterrence, in vagueness lies the best deterrence. How do you flesh out that vagueness? In repeated assurances by the United States that in the event act of aggression is committed against the Philippines, I don't believe going down into the details is the way the sincerity of the American commitment will be show," he said.

"They will respond depending on the circumstances but we are very assured, we are very confident that the United States in the words of Sec. Pompeo and in the words of Pres. Trump to our President: We have your back," he said.

President Rodrigo Duterte refused to flaunt Manila's victory before the Permanent Court of Arbitration and instead sought to repair economic and diplomatic ties with China.

The ruling was handed down a few months after Duterte took over from former President Benigno Aquino III, who initiated the arbitration proceedings.

During Chinese President Xi Jinping's state visit to the Philippines last November, Manila and Beijing entered into a memorandum of understanding to "arrive at an agreement" on gas in the disputed waters.