President Rodrigo Duterte has said martial law on Mindanao island could be extended across the Philippines, while the army fights Islamist militants.
As reported by the BBC, Mr Duterte said insurgents had beheaded a policeman on the southern island. At least three other members of the security forces are reported dead.
On Tuesday he imposed martial law for 60 days on Mindanao, where Muslim rebel groups are seeking autonomy.
Some of the groups have pledged support for so-called Islamic State (IS).