MANILA - Muslim militants holed up for nearly three weeks in Marawi, in southern Philippines, have been executing civilians trapped inside a bombed-out district in this lakeside city, officials said on Tuesday (June 13).

Lieutenant-Colonel Emmanuel Garcia, Civil Relations Group head of the Western Mindanao Command, said in a report that five civilians were gunned down on Monday by the militants linked to the Islamic State in Iraq and Syria (ISIS), who overran and seized large parts of Marawi on May 23.

The five civilians were hiding inside a house with 13 others when the gunmen came, according to the report. Five managed to escape. Eight were taken as hostages.

A video posted on the chat app Telegram by ISIS' Amaq news agency, meanwhile, showed the militants in Marawi executing six Christians. It was not clear when these men were killed, or who they were, amid reports that about 70 policemen in Lanao province, where Marawi is situated, remained missing.

"Based on revelations from trapped civilians we have recovered, (the militants) are using civilians as helpers, cooks, to carry their bullets and drag their wounded. This is what we're saying that civilians are being used as human shields," Lieutenant-Colonel Jay-Ar Herrera, spokesman of Task Force Marawi, said at a news briefing on Tuesday.

Rescuers said many of the hundreds still trapped in combat areas were dying of hunger.

Mr Zia Alonto Adiong, spokesman of a crisis management committee, said he received word that one child had died, and that the family had started eating blankets to survive.

Volunteers have been trying to extricate civilians caught in the crossfire via "peace corridors", but many of those trapped were still opting to hide rather than risk being discovered by the militants and executed.

Most, however, have run out of food and water.

An 81-year-old retired judge was close to dying of hunger and dehydration when he was rescued last week.

Almost the entire population of about 200,000 have already fled Marawi. But the military believes that beyond the checkpoints now fencing off the city's main roads, there were still some 500 to 600 civilians trapped or being held hostage.

Asked to comment on how much of the town was still occupied as the siege entered its fourth week, Lieutenant-General Carlito Galvez, head of the Western Mindanao Command, told Reuters it was 20 per cent - which is twice the area the military cited last week.

"Out of 96 barangays (neighbourhoods), they are holding portions in Marinaut, Lulut, Mapandi and Bongolo Commercial District, which only comprise 20 per cent of the whole Marawi city ... and it's getting smaller everyday," he said.

Brigadier-General Restituto Padilla had said a week ago that the fighters, from the ISIS-linked Maute and Abu Sayyaf groups, had been beaten back into just 10 per cent of the city.

On Tuesday, he estimated that about 100 militants were still fighting - down from the 400-500 believed to have stormed the town - but even with only "remnants" holding out, progress was slow for government troops because of the urban terrain.

"They will be obliged to move slowly because of the presence of potential civilians in the area who may be used as human shields," he said.

On Monday, Amaq claimed the Philippine military had "completely failed" to regain Marawi. "Islamic State fighters are spread in more than two-thirds of Marawi and tighten the chokehold on the Philippine army that is incapable of maintaining control of the situation," it said.

Brig-Gen Padilla branded the Amaq report "pure propaganda".

"Should we take their word that they control two-thirds of Marawi? With 202 confirmed terrorists killed why should we even give them the chance of airing their lies?" he said.

Heavy gunfire rang out in the downtown area of Marawi on Tuesday morning, as the military continued to target the militants with mortars and helicopter-mounted machine guns.

So far, 58 soldiers and policemen have died. Security officials said 202 militants have been killed.

Former military chief Rodolfo Biazon told ABC-CBN television on Monday that the government seemed to be struggling to control the situation in Marawi because its porous borders meant rebel forces could move freely in and out of the town.