Thousands of US, Philippine and Australian defence troops on Monday began annual drills to prepare to quickly respond to a range of potential crises, including in the disputed South China Sea.
The exercises have been opposed in recent years by China, which has territorial disputes in the South China Sea with several countries, including the Philippines, and suspects the drills are part of efforts to contain Beijing.
Washington and Manila say the drills are not directed against China, and that they also focus on responding to natural disasters and humanitarian crises. They will take place near the disputed South China Sea waters.
US Defence Secretary Ash Carter will fly to the Philippines to witness some of the 11-day exercises, underscoring the importance Washington puts on the joint combat drills that have been staged 32 times, said US Marine Lt Gen John Toolan, who heads the 5,000 American military personnel taking part in the manoeuvres.
Carter's presence will ''reaffirm that the relationship that we have with the Philippines is rock solid and we're side by side,'' Toolan, who heads US Marine forces in the Pacific, said at a news conference.
A highly mobile rocket system that has been deployed in hot spots such as Afghanistan will be used during the Balikatan, or Shoulder to Shoulder, exercises for the first time, he said.
''We are very, very expeditionary. We can move this stuff anywhere we need to,'' Toolan said.
This is the third year Australian personnel, including special forces commandos, will take part in the Balikatan drills.
Filipino military officials said a key exercise will involve US, Australian and Philippine forces retaking an oil rig seized by hostile units in a mock assault in an unused rig off the western province of Palawan, which faces the South China Sea.
The Philippines has turned to the United States, a long-time treaty ally, and others to rapidly acquire patrol ships and planes as its territorial rifts with China have escalated in the last four years. The disputes in the South China Sea also involve Vietnam, Malaysia, Taiwan and Brunei.
Two Japanese destroyers and a submarine docked at a Philippine port on 3 April near disputed South China Sea waters, where Beijing's increasingly assertive behaviour has sparked global concern.
''The Philippines is the least capable armed force in the region, and the US, being a big brother, is a big help,'' said Philippine Vice Admiral Alexander Lopez, who heads the contingent of about 3,500 Filipino military personnel involved in the exercises.
While many Filipinos welcome American support in strengthening the Philippines' territorial defence, left-wing activists and nationalists have opposed a growing US military presence in the former American colony, along with China's increasingly assertive advances in disputed waters.
Dozens of left-wing activists protested at the US Embassy in Manila, waving placards that read ''No to China aggression'' and ''US troops, Philippines is not your playground.''