Philippine president Ferdinand Marcos Jr has suggested his country's stand-off with China could spiral into war if a Filipino citizen were killed in what have been increasingly violent confrontations with the Chinese coast guard.

Asked how his government would react if Chinese coast guard actions caused the death of a Filipino serviceman, Marcos said: "If not only a serviceman but even a Filipino citizen is killed by a wilful act, that is very close to what we define as an act of war."