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.

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 United States unfairly by not paying more for defense.

Trump returned to the White House on Monday.

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.

Manila and Washington have deepened their defense cooperation since Philippine President Ferdinand Marcos took office in 2022 and began pushing back on Beijing's claims to the South China Sea.

Philippine Defense Secretary Gilberto Teodoro also met with new US National Security Advisor Mike Waltz at the White House on Wednesday to "discuss critical security matters," Manila's defense department said in a statement.

Teodoro "highlighted the importance of strengthening bilateral defense ties in addressing emerging geopolitical challenges," the statement said.

"Waltz and his team underscored the need to enhance cooperation with the Philippines and looked forward to increasing mutual security activities in support of a free and global Indo-Pacific (region)," it added.

Last month, the Philippines said it planned to acquire the US Typhon missile system as part of a push to secure its maritime interests, sparking warnings from China of a regional "arms race."