Al Haj Murad Ebrahim of the Moro Islamic Liberation Front told The Associated Press that intervening would have been difficult because President Rodrigo Duterte has declared his government would not negotiate with terrorists.

The nearly two months of insurrection in Marawi is the worst he has seen in his more than four decades as a rebel in the country's south, Mr Murad said.

He added the attackers have said through religious emissaries they are ready to fight to the death.

The violence underscores the urgency for the government and the Moro rebel group to implement a 2014 Muslim autonomy deal to help stop the rising tide of extremism, he said.

More than 550 people, including 413 militants, have been killed in Marawi since May 23, when hundreds of gunmen, waving Islamic State group-style black flags, stormed into the city centre, occupied commercial buildings and villages and took hostages.

Mr Duterte ordered troops to crush the attackers and declared martial law in the south for 60 days, which he now wants to extend up to the end of the year to deal with the worst crisis he has faced in his year-long presidency.

Amid the intense fighting that has forced nearly 400,000 people to flee from their homes, Mr Murad said his group met the president and offered to help rescue hundreds of residents, who have been trapped and starving in their homes in the battle zones.

Mr Duterte welcomed the offer, he said, and the government and the rebels worked to establish "peace corridors" through which trapped residents were extricated to safety.

A group of Muslim religious scholars managed to enter the conflict zone and urged some of the militant leaders to end the siege and withdraw from the beleaguered city.

The militants replied they would leave but that Mr Murad's group should intervene.

"I said it'll be hard for us to intervene because the president is very clear in his statements that he will not negotiate with the terrorists," Mr Murad told AP.

He said he asked the religious leaders to convince the gunmen to withdraw without any mediation, but they chose to fight it out.

"What they said, if nobody will intervene, then we will die here," Mr Murad said.

After weeks of setbacks, the militants have grown desperate, he said.

One of the leaders, Abdullah Maute, had a rift with another militant and killed him, while his brother, Omarkhayyam, had been wounded in battle and is no longer seen on the ground, he said.

Mr Murad's rebel group has a vast encampment in Butig town, near Marawi, and hundreds of its fighters have been ordered to stay in a designated zone to prevent them from being entangled in the clashes.

He travelled to Manila to witness government and rebel representatives submitting to Mr Duterte draft legislation that aims to establish a more powerful Muslim autonomous region in the south of the predominantly Roman Catholic nation.

The 69-year-old says he met late al Qaida leader Osama bin Laden when he was sent as a young insurgent for combat training in Afghanistan decades ago.

He said if the autonomy deal was set in place, the more moderate rebels in his group can provide crucial help in dealing with the smaller but radical groups that have emerged in the south.