Article of the 1951 MDT between the Philippines and the US indicates that:

"Each Party recognizes that an armed attack in the Pacific area on either of the Parties would be dangerous to its own peace and safety and declares that it would act to meet the common dangers in accordance with its constitutional processes.

Any such armed attack and all measures taken as a result thereof shall be immediately reported to the Security Council of the United Nations, Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security."

The American top envoy said he discussed this with President Rodrigo Duterte and Foreign Affairs Secretary Teddy Locsin Jr. during their meeting Thursday night.

Pompeo stressed that the disputed South China Sea is an important body of water for freedom of navigation.

"We remain committed to supporting not only the Philippines in that effort and the Philippines will need to do its part as well but all the countries in the region so that these incredibly vital economic sea lanes are open and China does not pose a threat to closing them down," Pompeo said.

The Trump administration has made it clear that it is committed to ensuring that the South China Sea remains open to all countries, as well as for commercial transit.

Read more at https://www.philstar.com/headlines/2019/03/01/1897776/us-assures-philippines-any-armed-attack-south-china-sea-will-trigger-mdt#EC8Z4Po5O9Bx1AYP.99