Manila and Washington on Thursday held talks on "concrete actions to address destabilizing activities" in Philippine waters.
"We discussed concrete actions to address destabilizing activities in the waters surrounding the Philippines including the West Philippine Sea," US Defense Secretary Lloyd Austin III told a briefing.
"And we remain committed to strengthening our mutual capacities to resist armed attack," he added.
Austin said he and his Filipino counterpart Carlito Galvez Jr. reaffirmed commitments under the decades-old Mutual Defense Treaty between the Philippines and the United States.
"We note the Mutual Defense Treaty applies to armed attacks on either of our armed forces, public vessels, or aircraft anywhere in the South China Sea or the West Philippine Sea," Austin explained.
The US will now have wide access to military camps in the Philippines after Manila agreed to designate four more Enhanced Defense Cooperation Agreement (EDCA) sites - to be added to the original five.
Washington has also vowed to help modernize the Philippine military.
Austin said these efforts are "especially important" as China "continues to advance its illegitimate claims" in the West Philippine Sea.
China has repeatedly rejected a 2016 landmark ruling by an international arbitral tribunal that invalidated its sweeping claims over the South China Sea, and recognized the Philippines' sovereign rights to areas within its exclusive economic zone.
Reacting to the US defense chief's statements, the Chinese Embassy said Austin "smeared China on the issue of South China Sea to advance the anti-China political agenda of the US."
The East Asian giant also said defense and security cooperation between countries should not target or harm the interests of a third party. It also warned that US efforts to step up military posture in the Asia-Pacific would only "escalate" tensions in the region.
"Such moves contradict the common aspiration of regional countries to seek peace, cooperation and development, and run counter to the common aspiration of the Filipino people to pursue sound economic recovery and a better life in cooperation with China," the Chinese Embassy said.
"It is hoped that the Philippine side stays vigilant and resists from being taken advantage of and dragged into trouble waters," it added.
The Philippine government has filed dozens of diplomatic protests against China over its activities and intrusions in the country's waters.
In early January, both President Ferdinand Marcos Jr. and Chinese leader Xi Jinping agreed to solve differences over the West Philippine Sea through peaceful means.
During his state visit to Beijing, Marcos also said he brought up the plight of Filipino fishermen to Xi, who promised to find a solution to avoid misunderstandings in the disputed waters.
