US and Philippine marines stormed a beach near a disputed rocky outcrop in the South China Sea on Friday as part of joint military drills involving more than 3,500 troops.
It is the first time the annual naval exercises have been held under Philippine President Ferdinand Marcos, who has expressed strong support for the decades-old alliance, after rocky relations under his predecessor Rodrigo Duterte.
Duterte had threatened to cancel drills and axe a key military deal with the United States as he pivoted towards China.
But Marcos told US President Joe Biden during a meeting in New York last month that he appreciated America's role in "maintaining the peace in our region".
China's recent war games around Taiwan, which it claims as part of its territory, rang alarm bells among nations surrounding the South China Sea.
Beijing claims sovereignty over almost the entire sea, while the Philippines, Vietnam, Malaysia and Brunei have overlapping claims to parts of it.
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China has ignored an international court ruling that its claims have no legal basis, and has aggressively asserted its stance.
It has built artificial islands as well as deployed hundreds of coast guard and maritime militia vessels to prowl the strategic waters, swarming reefs and harassing fishing and other boats.
The KAMANDAG exercises -- the Filipino acronym for "Cooperation of the Warriors of the Sea" -- kicked off Monday and will be held across the country's main island of Luzon until October 14.
One of the objectives is improving the Philippine military's coastal defence capability.
Around 300 troops were involved in Friday's amphibious drills held on an uninhabited beach in Zambales province, about 240 kilometres (150 miles) east of Scarborough Shoal, which China seized from the Philippines in 2012.
The rich fishing ground has become a flashpoint between the two countries.
"We are preparing for any threat that will be coming sooner or later," said Major Emery Torre, spokesman for the Philippine Marine Corps.
But Torre said the exercises did not simulate an attack by a particular country and were not related to a specific situation.
Marcos has taken a harder line on defending Philippine waters, insisting he would not let China trample on Manila's maritime rights.
During aerial surveillance over Scarborough Shoal on Thursday, the Philippine Coast Guard spotted six Chinese vessels, including four coast guard and two militia boats, in and around the small ring of reefs.
"We are doing the patrol operations to establish a presence at the area and also for the sake of our fishermen," said coast guard spokesman Armand Balilo.
As regional tensions rise, Washington is keen to preserve its security alliance with Manila, which includes a mutual defence treaty and permission for the US military to store defence equipment and supplies on several Philippine bases.
It also allows US troops to access certain military bases in the country.