The United States has reiterated its stance that it will defend the Philippines if it comes under attack by China.
About 220 Chinese fishing vessels were found moored near a coral reef about 300 kilometers west of Palawan Island in the South China Sea in early March. Some of them are still there.
Manila has lodged a protest with Beijing stating that the waters are inside its exclusive economic zone.
During a press briefing on Wednesday, US State Department spokesperson Ned Price said the US is taking the view that Chinese maritime militia personnel are aboard the vessels that have remained there.
He also said that an armed attack against the Philippines' armed forces in the Pacific, including in the South China Sea, will trigger the country's obligations under the US-Philippines Mutual Defense Treaty.
He went on to urge China to abide by the 2016 arbitral tribunal award under the Law of the Sea Convention, which he said is final and legally binding on all parties.
The administration of President Joe Biden has indicated that it will step up its involvement in the South China Sea as it strengthens cooperation with allies, including Japan and the Philippines.