ISIS fighters in the Philippines are forcing women to become sex slaves and hiding behind 100 Christian human shields as the vicious battle to retake a city held by the militants enters its second month.

Terrorists from the Maute group seized Marawi City five weeks ago and have taken hostages who they are forcing to convert to Islam and carry wounded fighters.

Most of Marawi's 200,000 citizens have fled since the attack on May 23, and those left behind have been subjected to horrific abuse, according to escapees. 

'They are being forced to be sex slaves, forced to destroy the dignity of these women,' said army spokesman Jo-Ar Herrera.

'This is what is happening inside, this is very evident..these are evil personalities.'

Hostages include leading churchman Father Teresito Soganub. 

Abdullah Maute, one of two brothers who formed the group, said on Sunday he would free Soganub in return for his parents, who are being held by police. 

But this request was refused by Philippines president Rodrigo Duterte, with his spokesman saying any bargaining with terrorists was against government policy. 

Heavy clashes broke out today as strongman leader Duterte ordered intense bombing raids by planes on the shrinking rebel zone. 

The authorities believe Soganub and more than 100 other Christians are being held by the ISIS-affiliated insurgents as human shields.

Taking advantage of a short truce on Sunday to mark the Eid al-Fitr, eight Muslim leaders entered the conflict zone and met briefly with Maute. 

This meeting was condemned by the Philippine government, who said it was sticking to its policy of not negotiating with the terrorists. 

A spokesman said: 'Let us remind the public, the gravity of the terrorists and their supporters' offences is immense and they must be held accountable for all their actions.' 

The Maute brothers' father, Cayamora Maute, was arrested on June 6 in Davao City, about 140 km southeast of Marawi. 

Their influential businesswoman mother, Farhana Maute, was arrested three days later, closer to Marawi.

The military's public relations machine has been insisting the rebel leadership was crumbling, saying top commanders had escaped or been killed.

It said the rebels had executed some of their own men for wanting to surrender.

And authorities claimed Isnilon Hapilon, ISIS's anointed 'emir' in Southeast Asia, had fled the warzone and 'abandoned his comrades'.

Military officers, however, have said they lack solid proof of such developments and were working to verify intelligence reports.

The protracted siege on Marawi has caused concerns about the strength of ISIS in the southern Philippines.

The rebels' organisation, combat capability and use of heavy weapons has raised fears in the mainly Roman Catholic country that it is unprepared for the threat.

Fighting has raged in the town since an operation to arrest Hapilon went wrong on May 23, leading to the government losing not just Hapilon, but control of the city.

Some 70 servicemen, 27 civilians and 290 militants have since been killed  and 246,000 people have been displaced from a town where some neighbourhoods have been reduced to rubble.

Fleeing residents believe large numbers of civilians may have been killed while trapped in areas under fire from militant snipers and battered by artillery and air strikes.