Thousands of US and Filipino marines launched 10 days of joint exercises in the northern and western Philippines on Tuesday, a day after China held huge drills around Taiwan.
The annual Kamandag, or Venom, exercises are focussed on defending the north coast of the Philippine's main island of Luzon, which lies about 800 kilometres (500 miles) from self-ruled Taiwan.
Beijing considers Taiwan part of its territory and has vowed it will never rule out using force to take it, calling Monday's drills a "stern warning" to "separatist" forces on the island.
The joint US-Filipino exercises come amid a series of escalating confrontations between China and the Philippines over reefs and waters in the South China Sea, which Beijing claims almost in its entirety.
Philippine Marine Corps commandant Major-General Arturo Rojas stressed at Tuesday's opening ceremony in Manila that Kamandag was long planned and had "nothing to do with whatever is happening in the region".
The drills' primary focus will be live-fire exercises along Luzon's north coast, while other activities will be conducted on tiny Philippine islands between Luzon and Taiwan.
"It's a coastal defence doctrine. The doctrine says that a would-be aggressor might be directed towards our territory," Filipino exercise director Brigadier-General Vicente Blanco told reporters.
"We are not exercising to join the fight (over Taiwan)," he added.
US Marines representative Colonel Stuart Glenn said the exercises were aimed at helping the United States and its allies respond to "any crisis or contingencies".
The western Philippine island of Palawan, facing the disputed South China Sea, will also host part of the drills.
The US and Philippines are fielding just over a thousand participants each, while smaller numbers of Australian, British, Japanese and South Korean forces are also taking part.
An amphibious landing and training on how to defend against chemical and biological warfare were also among the activities planned, according to a press kit.
As the war games began Tuesday, the Philippine government announced that one of its civilian patrol vessels had sustained minor damage on October 11 when it was "deliberately sideswiped" by a "Chinese Maritime Militia" vessel.
The Bureau of Fisheries and Aquatic Resources said the collision, which dented the front right section of the BRP Datu Cabaylo, took place about 9.3 kilometres (5.8 miles) from Thitu, a Philippine-garrisoned island in the Spratly group.
The crew were unhurt and later sailed the vessel to Thitu and completed their routine maritime patrol mission, the statement said.
Beijing has for years sought to expand its presence in contested areas of the sea, brushing aside an international ruling that its claim to most of the waterway has no legal basis.
China has deployed military and coast guard vessels in recent months in a bid to eject the Philippines from a trio of other strategically important reefs and islands in the South China Sea.
