The death toll in a battle over the southern city of Marawi, 800 kilometres south of Manila, has reached more than 100, including 18 civilians.
Among the civilians killed, eight were men were believed to have been executed and then thrown into a ravine along a highway.
The eight dead men, most of them shot in the head and some with hands tied behind their backs, were labourers who were stopped by Islamic State-linked militants on the outskirts of Marawi City while trying to flee clashes, according to police.
The discovery confirms days of speculation that Maute rebels had killed civilians during a bloody takeover of Marawi City, that the military believes is aimed at winning the Maute recognition from the Islamic State group in the Middle East as a Southeast Asian affiliate.
The army deployed additional ground troops over the weekend and dispatched helicopters to carry out rocket strikes on Maute positions as fighters held buildings and a bridge deep inside a predominantly Muslim city where few civilians remained.
Marawi police officer Jamail C Mangadang told Reuters the eight men found dead were carpenters who were part of an evacuation convoy stopped by rebels late on Saturday.
Recalling information provided by their manager, Mangadang said the victims were pulled off a truck because they were unable to cite verses of the Koran, the Islamic Holy text.
"We heard gunfire, although I'm not sure if it was the same people who were shot," he said at the scene.
"Early in the morning, at 08.20, there are civilians, concerned citizens, who said 'can you verify these dead bodies?'."
Tens of thousands of people have fled Marawi since Tuesday, when militants went on the rampage seizing a school, a hospital, and a cathedral.
Christians were taken hostage, according to church leaders, and more than 100 inmates, among them militants, were freed when rebels took over two jails.
The violence erupted in the moments after a failed attempt by security forces to capture Isnilon Hapilon, a leader of a radical faction of another extremist group, who the government believes is Islamic State's point-man in the Philippines.
The military is certain the Maute are protecting Hapilon and had narrowed down his location. Hapilon leads a radical faction of another Mindanao-based group, the Abu Sayyaf.
The little-known Maute group has staged similar, days-long sieges on Mindanao island but none on the scale of Marawi, where witnesses said flags resembling those of Islamic State had been flown and some men were wearing black headbands.