The army fired artillery rounds and rockets on Monday to separate warring rebel factions fighting for control of a river delta in Datu Piang town, prompting a rebuke from the Moro Islamic Liberation Front (MILF) that it had breached a cease-fire.
A truce has been holding for about a year on Mindanao island. The rebels have said they are concerned about the new government's plan to overhaul talks to end the long-running insurgency.
"We filed a protest," Mohagher Iqbal, the rebels' chief peace negotiator, told reporters from the guerrillas' main base. "It's a violation of our cease-fire. Even if our men were fighting each other, it does not justify the military's involvement."
The army said more than 5,000 people had fled their homes and farms since the violence first erupted on Saturday. Maj. Gen. Anthony Alcantara, the most senior army commander in central Mindanao region, dismissed speculation the intention was to provoke fresh hostilities with the rebels.
"We're not operating against them, we're only monitoring the developments and helping the displaced people," Alcantara told Reuters, saying the army had not violated the truce.
Iqbal said the army should have allowed the MILF to resolve the conflict after it suspended two warring field commanders to defuse tension ahead of Ramadan, which starts on Wednesday.
Philippine governments and the MILF have been in stop-start negotiations since 1997, but an acceptable political deal remain elusive.
