US Defense Secretary Lloyd Austin has stressed Washington's defense commitments to Manila following Sunday's collisions between Philippine and Chinese vessels in the South China Sea.

The Pentagon said on Friday that Austin discussed the incident with Philippine Defense Secretary Gilberto Teodoro over the phone the previous day.

The defense chiefs called the maneuvers by the China Coast Guard and maritime militia that obstructed a routine Philippine resupply mission to Second Thomas Shoal "dangerous and unlawful."

They also said the collisions put the safety of Philippine vessels and crew at risk.

The Secretaries reaffirmed that the Mutual Defense Treaty extends to public vessels, aircraft, and armed forces anywhere in the Pacific, including the South China Sea.

The Pentagon said Austin will take part in an expanded defense ministers' meeting of the Association of Southeast Asian Nations to be held in the Indonesian capital of Jakarta in November.

Washington is stepping up vigilance as it suspects China may be trying to change the status quo by force over Second Thomas Shoal where the Philippine military has an outpost.