Police Special Action Force commandos and army troops were patrolling on Talomo mountain outside Davao city on Mindanao island when they encountered at least 30 New People's Army (NPA) rebels, triggering the gunbattle yesterday.
"There had been reports of NPA harassment in the area, including extortion activities, and that was why they went on a routine security patrol," regional military spokesman Captain Alberto Caber told AFP.
"The troops caught up with the NPA unit, touching off a 30-minute firefight," he said.
The NPA is the armed unit of the Communist Party of the Philippines, which has been waging a Maoist insurgency since 1969.
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The NPA rebels are known to extort money from politicians, businessmen, mining firms and community members to fund their operations.
Failing to pay up could result in retaliation, including targeted killings.
NPA guerrillas over the past two months killed two Mindanao mayors in roadside ambushes.
President Benigno Aquino had hoped to reach a peace deal with the communist rebels before his six-year term ends in 2016, but planned peace talks have been hampered by rebel demands that detained comrades be freed.