Washington DC - Secretary of State Marco Rubio said Wednesday the US under President Donald Trump remained committed to the Philippines' defense as tensions simmer with Beijing in the South China Sea.

In a call with his Philippine counterpart Enrique Manalo, Rubio "underscored the United States' ironclad commitments to the Philippines under our Mutual Defense Treaty," State Department spokeswoman Tammy Bruce said.

Rubio, a longtime hawk on China, discussed the "dangerous and destabilizing actions in the South China Sea" by Beijing, formally known as the People's Republic of China (PRC).

"Secretary Rubio conveyed that the PRC's behavior undermines regional peace and stability and is inconsistent with international law," Bruce said.

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US leaders have repeatedly stood by the Philippines, a treaty ally and former US colony.

But Trump is known for questioning alliances, including NATO, claiming allied nations treat the US unfairly by not paying more for defense.

Rubio made the call with his Philippine counterpart a day after a veiled warning to Beijing on the South China Sea during a four-way meeting with his counterparts from India, Japan, and Australia.

The Philippines have engaged in increasingly tense confrontations with China over disputed South China Sea waters and reefs over the past year.

China claims most of the strategic waterway despite an international tribunal ruling that its claim lacked any legal basis.