The military believes that civilians trapped in Marawi are being used as sex slaves by terrorists, as the bloody occupation of the city enters its sixth week.
It is also feared that Christian hostages are being forced to convert to convert to Islam, loot homes and take up arms against government troops.
Speaking at a news conference today (Tuesday, June 27), military spokesman Jo-Ar Herrera said: “So they are being forced to be sex slaves, forced to destroy the dignity of these women.
“So this is what is happening inside, this is very evident… these are evil personalities.”
Mr Herrera said the fears were based on the accounts of people who had managed to escape or been rescued from the clutches of the Maute group.
Although impossible to verify, some escapees also describe numerous bodies being left to rot in the streets of the war-torn city.
The strength of the Islamist forces and an apparent influx of foreign fighters have stoked concerns that the Marawi crisis might be only the start of a wider campaign to create a new stronghold, or caliphate, on the island of Mindanao.
Continuing heavy clashes have been reported today, with intensive aerial bombardment on territory controlled by militants.
The government has ruled out negotiating with the terrorists after it was reported that Abdullah Maute offered to trade captive priest Father Teresito Suganob for his mother Farhana and father Cayamora, who were arrested in separate operations this month.
However, presidential spokesman Ernesto Abella said deals with militants were against government policy.
“The local religious leader-led talks with terrorists last Sunday was one not sanctioned,” he told reporters.
“Any demands made inside, therefore, hold no basis. Let us remind the public, the gravity of the terrorists and their supporters’ offences is immense.”
Yesterday, the military claimed that the terror group’s leadership was crumbling, and that some top commanders had fled or been killed in action.
It is also investigating claims that Isnilon Hapilon, IS’s regional representative, or “emir”, had fled the city.
“It would be a clear sign of his cowardice,” Mr Abella said. “It may only be a matter of time before they disintegrate.”
Fighting has raged in the town since an operation to arrest Hapilon went wrong on May 23. It is believed the Abu Sayyaf commander was in the city on a mission to unite the numerous islamist groups of the Philippines under the black flag of IS.
The strength of the forces that erupted after the failed raid, along with the presence of foreign jihadis, suggests he made some headway toward this aim.
Official figures say 70 servicemen, at least 27 civilians and 290 militants have been killed and 246,000 displaced.