Ten suspected militants were killed in the Philippines on Monday as they attempted to infiltrate a southern city partly occupied by gunmen loyal to the Islamic State group, officials said.

Government forces have been besieging the militants in the city of Marawi for almost 100 days. But the gunmen, flying the IS black flag, have defied military assaults including airstrikes and artillery barrages.

Before dawn Monday 10 militants aboard two motorboats were caught by soldiers trying to sneak into the lakeside city to reinforce gunmen already there, said regional military chief Lieutenant General Carlito Galvez.

The bodies of five dead men were recovered from one motorboat while the other boat, with five aboard, was sunk, Galvez said.

"Let this be a warning to those who have the intention to escape and to enter (Marawi), our troops are ready for you. We will definitely get you," he added.

Troops have previously intercepted dozens of "reinforcements" and ammunition being brought into the city to help the militants.

Aside from the forces in Marawi, various Muslim outlaw bands are scattered throughout the strife-torn southern Philippines, engaged in kidnappings for ransom, banditry and Islamic separatist guerrilla activities.

The military says that so far, 603 militants, 45 civilians and 130 soldiers have been killed in the fighting, which has left the once-thriving city of Marawi looking like war-torn Aleppo or Mosul.

It says the gunmen, led by the so-called Maute group that has pledged allegiance to IS, have recently been driven out of strategic areas like the city's main mosque and a cathedral, and been reduced to a small band occupying less than a square kilometre of the city.

But their use of improvised explosives and booby traps as well as the difficulty of urban combat has slowed down the military's progress.

Military chief General Eduardo Ano said Monday the remaining militants were "consolidating" and preparing for the military's assault.

"Our enemy has said they will fight it out up to their last breath so with that declaration, we expect they will really do or die," he told reporters in Manila.

The death of patriarch Cayamora Maute in custody on Sunday may demoralise the remaining fighters, he added.

Cayamora Maute was arrested in June and is suspected of having assisted his sons, the leaders of the Maute group, in their illegal activities.

President Rodrigo Duterte declared martial law over the entire southern Philippines on May 23 after the fighting broke out in Marawi.