Islamists in the besieged city of Marawi in southern Philippines are forcing female hostages into sex slavery, Manila's army has said. Male and female captives are also made to assist the militants in their fight against the Filipino military.

Marawi city in the Mindanao island turned into a centre of conflict following President Rodrigo Duterte's declaration of martial law in the southern Philippines in May. The military is hoping to fully rid the island of militants belonging to the Maute group in the coming days. However, the Isis-inspired extremists are exhibiting stiff resistance.

"Worst thing [is] there are cases of female hostages forced to marry the Maute local terrorist group. They are being forced to [be a] sex slave, forced to destroy the dignity of these women," Joint Task Force Marawi spokesperson Lieutenant Colonel Jo-ar Herrera told reporters.

Herrera cited examples of hostages who either escaped or were rescued and said their accounts show that women suffered brutal sexual assaults at the hands of the Maute captors. The captives were also tasked with plundering abandoned houses in the conflict-ridden area to fuel the Maute insurgency.

"The hostages were tasked to loot houses, establishments [for] ammunition, firearms, cash, [and] gold," added Herrera.

Soon after clashes erupted in the island between the government forces and militants, many of the 200,000 inhabitants fled Marawi. Those who remained trapped in the region are often used as human shields by the militants. Earlier reports also said Marawi residents, facing an acute food shortage, had resorted to eating leaves and cardboard boxes for their survival.

"This is what is happening inside, this is very evident. These are evil personalities," said Herrera referring to the Maute Islamists. The army's claims of militants using women as sex slaves have not yet been independently verified.

Meanwhile, Duterte has said his government is determined to rebuild Marawi once the violence subsides. Emerging after a long absence in public, he said: "I know the deployment of snipers and where they hid their firearms. I already had the complete picture and I knew that would be a long fight. What's painful for me, a fractured ideology entered. All that they want is to kill and destroy, how can we live with that?"

Philippines Marawi sex slaves
Muslim women walk after praying during Eid al-Fitr at a mosque inside city hall compound in Marawi City, Philippines Jorge Silva/Reuters