General Romeo Brawner made the comments as Beijing's military surrounded Taiwan in large-scale exercises it said was practice for a blockade of the self-ruled island which China has vowed to one day bring under its control.

"Start planning for actions in case there is an invasion of Taiwan," he told troops in northern Luzon island.

"Because if something happens to Taiwan, inevitably we will be involved," Brawner said without naming the potential invader.

"As members of the armed forces of the Philippines, we should have that mentality that we are already at war."

Part of his soldiers' job would be "to rescue" a quarter million Filipinos working in Taiwan, Brawner said without detailing how the military would execute that mission.

The northern Philippines will play host to large-scale joint exercises with ally the United States scheduled to begin April 21, Brawner told troops at Northern Luzon Command headquarters.

"These are the areas where we perceive the possibility of an attack. I do not want to sound alarmist, but we have to prepare," he added.

Manila, which has a mutual defence pact with Washington, has been engaged in months of confrontations with Beijing over disputed areas of the South China Sea.

Beijing claims almost the entirety of the crucial waterway, despite an international ruling that its assertion has no merit.

US Defence Secretary Pete Hegseth visited Manila last week and said that "friends need to stand shoulder to shoulder to deter conflict to ensure that there's free navigation" in the South China Sea.

At this month's Balikatan exercises, US and Philippine troops will conduct a "full battle test" of "all of the plans, all of the doctrines, all of the procedures that we have developed in the past years", Brawner said.

"It's very important that we prepare for any eventuality," he added.

Brawner alleged that "communist China is already conducting united front works in our country".

"They are already infiltrating our institutions, our schools, our businesses, our churches, even our ranks in the military," he said without giving details.

He said the Philippines was also experiencing "cyber warfare, information warfare, cognitive warfare, political warfare".

The Chinese embassy did not immediately respond to requests for comment.

The Philippines has in recent weeks made a series of arrests of Chinese suspects accused of conducting surveillance on military camps and Filipino naval and coast guard ships.