The war games will feature live-fire exercises in the north of the Philippines facing the Taiwan Strait, as well as a province off the disputed South China Sea, where Philippine and Chinese forces have engaged in repeated confrontations.
In one drill, the Japanese military, which is contributing about 1,400 personnel, will use a Type 88 cruise missile to sink a World War II-era minesweeper off the coast of northern Luzon island.
More than 17,000 soldiers, airmen and sailors are taking part in the 19-day Balikatan, or "Shoulder to Shoulder," exercises -- about the same number as last year's edition, including contingents from Australia, New Zealand, France and Canada.
Balikatan comes as Iran and the United States, along with Israel, edge towards the end of the two-week ceasefire that halted the Middle East war, ignited by surprise US-Israeli strikes on the Islamic republic.
"Regardless of the challenges elsewhere in the world, the United States' focus on the Indo-Pacific and our ironclad commitment to the Philippines remains unwavering," US Lieutenant General Christian Wortman said at Monday's opening ceremony.
Without providing precise numbers, Wortman, commander of the Marine Expeditionary Force, later told reporters that approximately 10,000 US personnel would take part in the exercises.
Philippine military chief General Romeo Brawner added that US Indo-Pacific Command chief Admiral Samuel Paparo had assured him at the war's outbreak that this year's Balikatan would be "the biggest ever".
Among the high-end weapons expected to be used is a US Typhon missile system that has been in the archipelago since visiting US forces left it there in 2024, provoking outrage from Beijing.
"We anticipate that it will be incorporated at some level during the course of the exercise," Wortman said.
- 'Playing with fire' -
While both militaries insisted that no exercises would take place "near Taiwan", coastal defence drills are set fewer than 200 kilometres (120 miles) from the island's southern coast.
Beijing has ramped up military pressure around self-ruled Taiwan, which it considers part of its territory and has threatened to use force to seize.
China slammed the joint exercises on Monday, saying the United States, Japan and the Philippines were "playing with fire".
"What the Asia-Pacific region needs most is peace and tranquility, and what it needs least is the introduction of external forces to sow division and confrontation," Chinese foreign ministry spokesman Guo Jiakun told a news briefing.
"We wish to remind the countries concerned that blindly binding themselves together in the name of security will only be akin to playing with fire -- ultimately backfiring upon themselves," he added.
Philippine President Ferdinand Marcos warned in November that given his country's proximity to the island democracy, "a war over Taiwan will drag the Philippines, kicking and screaming, into the conflict."
In February, US, Japanese and Philippine aircraft patrolled over the Bashi Channel that separates the Philippines from Taiwan to test what Manila called their "ability to operate seamlessly together in complex maritime environments".
Japan's first Balikatan as a full participant follows the signing of a reciprocal access agreement approved by the Japanese Diet last June.
Colonel Takeshi Higuchi of Tokyo's joint staff told Japanese media the drills would "contribute to creating a security environment that tolerates no attempt to unilaterally change the status quo by force".
Marcos has been building up security ties with Western nations to deter China. Over the past two years, Manila has also signed visiting forces or equivalent agreements with New Zealand, Canada and France to facilitate joint military exercises.
Outside the Manila base where Monday's opening ceremony was held, a group of about 50 people protested against the exercises, holding aloft signs branding US President Donald Trump an "imperialist terrorist" and demanding US forces leave the country.