Blinken made the remarks in a call with Philippine Foreign Minister Teodoro Locsin, the U.S. State Department said in a statement, in which he also stressed the importance of a long-standing defence agreement between the allies and its clear application if Manila came under attack in the South China Sea.
"Secretary Blinken pledged to stand with Southeast Asian claimants in the face of PRC pressure," it said, referring to the People's Republic of China.
Blinken's assurances came after Locsin said on Wednesday the Philippines had filed a diplomatic protest over China's passing of a law allowing its coastguard to open fire on foreign vessels, describing it as a "threat of war".
China passed the legislation on Friday allowing its coastguard to use "all necessary means" to stop or prevent threats from foreign vessels, including demolishing other countries' structures built on Chinese-claimed reefs.
China claims almost all of the energy-rich South China Sea, which is also a major trade route. The Philippines, Brunei, Vietnam, Malaysia and Taiwan have overlapping claims.
The United States, which has accused China of taking advantage of the distraction of the coronavirus pandemic to advance its presence in the South China Sea, has sent a carrier group through the waterway to promote "freedom of the seas".
China on Tuesday said it would hold military drills of its own this week.
For an optimal experience, please
