Thousands of allied American and Filipino forces have opened annual combat drills that are set to include repelling an island attack to simulate the defence of the Philippine archipelago and seas in a "full-scale battle scenario" that has antagonised China.

The annual Balikatan military exercises between the longtime treaty allies are scheduled from April 21 to May 9 with about 9000 American and 5000 Filipino military personnel.

Fighter jets, warships and an array of weaponry including a US Marine anti-ship missile system will be involved, US and Philippine military officials said.

China has steadfastly opposed such war drills in or near the disputed South China Sea and in northern Philippine provinces close to Taiwan, especially if they involve US and allied forces that Beijing says aim to contain it and, consequently, threaten regional stability and peace.

"We are ready," US Marine Corps Lieutenant General James Glynn said when asked if US and Philippine forces had built up the capability to address any major act of aggression in the Taiwan Straits or the South China Sea after years of joint combat exercises.

"Our combined strength … possesses a degree of lethality for a force that possesses an indomitable warrior ethos and spirit," Glynn said in a speech in the opening ceremony of the annual combat-readiness exercises.

"It's all dedicated to one purpose, to ensure the defence of the Philippines and to maintain a free and open Indo-Pacific."

"All of us want to resolve any regional conflict peacefully but should deterrence fail, we need to be prepared," said Glynn. 

He previously helped lead special operations forces against the Islamic State and served in Fallujah, Iraq.

Philippine army Major General Francisco Lorenzo said the exercises during Balikatan, which means shoulder to shoulder in Tagalog, are not aimed at any particular country.

"It's joint training with the US forces to increase our capability in securing our territory and, of course, it will increase our capabilities and our preparedness and responsiveness to any eventuality," Lorenzo said.

The exercises are set to include a mock allied counter-assault against an enemy attack on an island, the use of a barrage of artillery and missile fire to sink a mock enemy ship, joint navy sails in or near the disputed South China Sea and aerial combat surveillance, according to the Philippine military.

Glynn described this year's drills as "full battle tests" where capabilities of both forces will be measured in multiple scenarios. 

"The full battle tests is intended to take into consideration all of the regional security challenges that we face today, beginning in the South China Sea," Glynn said.

A Philippine military statement described this year's large-scale combat exercises as "a full-scale battle scenario meticulously designed to rigorously test and enhance the combined capabilities of both nations' armed forces under the most realistic and challenging conditions."

Aside from US and Philippine military personnel, Australia plans deploy about 260 participants, an Australian military officer said.

Several countries including Japan intend to send military observers.

A mid-range missile system, which was deployed to the northern Philippines by the US Army last year, will be used again in the combat exercises, US and Philippine military officials said without offering additional details.

China has repeatedly expressed alarm over the missile deployment and demanded the Philippines pull the weaponry from China's territory, which it said could spark an arms race.

The US Army's mid-range missile system consisting of a mobile launcher and at least 16 Standard Missile-6 and Tomahawk Land Attack Missiles has been repositioned in the Philippines, a Philippine official said earlier this year.

The system was repositioned from the northern Philippine city of Laoag to a strategic area in a western coastal province facing a disputed South China Sea shoal, where Chinese and Philippine coast guard and navy forces have had increasingly tense confrontations.

US Defense Secretary Pete Hegseth visited Manila last month in his first trip to Asia and said the Trump administration would work with allies to ramp up deterrence against China's aggression in the South China Sea.

The US was not gearing up for war, Hegseth said, while underscoring that peace would be won "through strength".

During the Balikatan exercises, the US would deploy an anti-ship missile system called the Navy Marine Expeditionary Ship Interdiction System, as well as unmanned sea vessels to enable the allies forces to train together to defend Philippine sovereignty, Hegseth said.

The allied forces also agreed to stage special operations forces training in Batanes province in the northernmost tip of the Philippine archipelago across a sea border from Taiwan, he said.

Aside from China and the Philippines, Vietnam, Malaysia, Brunei and Taiwan also have overlapping claims in the busy waterway.

Washington does not lay any claim to the waterway but repeatedly has warned of an obligation to defend the Philippines, its oldest treaty ally in Asia, if Filipino forces, ships and aircraft come under an armed attack, including in the South China Sea.