US and Filipino troops fired missiles and artillery at an imaginary "invasion" force during war games on the Philippines' northern coast Monday, days after their governments objected to China's "dangerous" actions in regional waters.

Thousands of troops are conducting land, sea and air manoeuvres against a backdrop of increased confrontations between Chinese and Filipino vessels around shoals in the South China Sea claimed by Manila, as well as stepped-up Chinese air and naval activity around nearby self-ruled Taiwan.

US troops massed at a strip of sand dunes on Luzon island's northwest coast -- around 400 kilometres (249 miles) south of Taiwan -- let loose more than 50 live 155mm howitzer rounds at floating targets about five kilometres off the coast, AFP journalists saw.

Filipino troops followed up by firing rockets aimed at wearing down the attackers, before the two forces finished the job with machine guns, Javelin missiles and more artillery rounds.

Lieutenant General Michael Cederholm, commander of the US First Marine Expeditionary Force, said the exercise was "to prepare for the worst" by "securing key maritime terrain".

"It's designed to repel an invasion," Cederholm told reporters at the exercise site. 

"Our northwestern side is more exposed," Major General Marvin Licudine, exercise director for the Filipinos, told AFP ahead of the live firing at the La Paz sand dunes near Laoag city.

"Because of the regional problems that we have... we have to already practise and orient ourselves in our own land in these parts," he added.

Beijing claims almost the entire South China Sea despite an international ruling that its assertion has no legal basis.

It deploys hundreds of coast guard, navy and other vessels to patrol and militarise the waters.

Asked about Monday's drills, China's foreign ministry warned that "any military exercise should not be targeted at or harm the interests of third parties".

"Countries in the region can all see clearly who is currently provoking military confrontation and escalating tensions in the region," spokesman Lin Jian told a regular briefing.

Just last week, Manila said the China Coast Guard damaged a Philippine Coast Guard ship and another government vessel in water cannon attacks around the disputed China-controlled Scarborough Shoal in the South China Sea on April 30.

More than 16,700 Filipino and American troops are taking part in the annual military drills -- dubbed Balikatan, or "shoulder to shoulder" in Tagalog -- in multiple locations across the Asian archipelago.

'Destabilizing conduct'

Maritime confrontations between China and the Philippines have raised fears of a wider conflict that could involve the United States and other allies.

Monday's exercise came days after the defence ministers of the Philippines, the United States, Japan and Australia met in Hawaii and issued a joint statement on their strong objections to the "dangerous and destabilising conduct" of China in the South China Sea.

The ministers "discussed opportunities to further advance defence cooperation" and to "work together to support states exercising their rights and freedoms in the South China Sea".

Last week, US forces taking part in the Balikatan exercises fired HIMARS precision rockets into the South China Sea from the western island of Palawan, the nearest major Philippine landmass to the hotly disputed Spratly Islands.

The US Marine Corps said the manoeuvre was a rehearsal for the rapid deployment of the missile system across the Philippines' South China Sea coast to "secure and protect Philippines' maritime terrain, territorial waters and exclusive economic zone interests".

The confrontations between the Philippines and China comes as tensions have ratcheted up between Beijing and Taipei, which is about to inaugurate a new president regarded by China as a dangerous separatist.

Taiwan's defence ministry said Friday it had detected 26 Chinese aircraft and five naval vessels around the self-ruled island in the previous 24 hours.

"To a degree, military exercises are a form of deterrence," Philippine Foreign Secretary Enrique Manalo was quoted as saying in remarks delivered on his behalf by an aide at a public workshop on Friday.

"The more we simulate, the less we actuate," he added.