Responding to a request for comment, a spokeswoman for the US State Department reiterated a March 28 statement by Secretary of State Antony Blinken saying the US "stands with our ally, the Philippines, in the face of (China's) maritime militia pressure in the South China Sea."

"As we have stated before, an armed attack against the Philippine armed forces, public vessels or aircraft in the Pacific, including in the South China Sea, will trigger our obligations under the US-Philippines Mutual Defense Treaty," the spokeswoman added.

China claims almost the entire South China Sea, through which about $3 trillion of shipborne trade passes each year. In 2016, an arbitration tribunal in The Hague ruled the claim was inconsistent with international law.

In a statement on Monday, the Philippine Foreign Ministry accused China's coast guard of "shadowing, blocking, dangerous maneuvers, and radio challenges of the Philippine coast guard vessels."

On Sunday, the Philippines vowed to continue maritime exercises in its South China Sea EEZ in response to a Chinese demand that it stop actions it said could escalate disputes.

As of April 26, the Philippines had filed 78 diplomatic protests to China since President Rodrigo Duterte took office in 2016, Foreign Ministry data shows.

"Our statements are stronger too because of the more brazen nature of the activities, the number, frequency and proximity of intrusions," said Marie Yvette Banzon-Abalos, executive director for strategic communications at the Foreign Ministry.

Duterte, for the most part, has pursued warmer ties with China in exchange for Beijing's promises of billions of dollars in investment, aid and loans.

"China remains to be our benefactor. Just because we have a conflict with China does not mean to say that we have to be rude and disrespectful," Duterte said in a weekly national address.

"So, kindly just allow our fishermen to fish in peace and there is no reason for trouble," Duterte said, addressing China.