Abu Sayyaf insurgents killed six soldiers in the southern Philippines on Sunday despite a new military offensive against the Al Qaeda-linked group, a military spokeswoman said.
The soldiers were patrolling in the strife-torn island of Basilan when they were attacked by about 20 heavily-armed fighters.
Regional military spokeswoman Captain Rowena Muyuela said there were also "most likely... undetermined casualties on the Abu Sayyaf side".
The incident came despite a military offensive that was launched over the weekend against the Abu Sayyaf in the island of Jolo, 129 kilometres (80 miles) from Basilan.
On Saturday troops battled about 60 Abu Sayyaf fighters in Jolo and even used helicopter gunships to blast their positions, the military said.
Muyuela could not say if the latest attack in Basilan was related to the violence in Jolo.
The two heavily-forested islands are known strongholds of the Abu Sayyaf, which was set up in the 1990s with seed money from Osama bin Laden's Al-Qaeda network.
It has been blamed for the worst terror attacks in the country's history including bombings and mass kidnappings of Christians and foreigners.
It has engaged in extortion and banditry, demanding huge ransoms for its kidnap victims.
The Abu Sayyaf recently pledged allegiance to the Islamic State group, although the military and government officials say this is merely a propaganda move to boost their image.