Philippine President Rodrigo Duterte has sent more troops to fight Muslim militants holding out for a fifth week in the southern city of Marawi, rejecting offers from religious leaders and local officials to mediate to get the remaining gunmen to surrender.

This came as military spokesman Restituto Padilla said yesterday that the push towards the last remaining strongholds of the militants has slowed to a crawl because soldiers are going "door to door, inch by inch".

More than a hundred gunmen from militant groups are estimated to be still holding out in Marawi.

The pitched battle began on May 23, when Islamist radicals from the Abu Sayyaf and Maute militant groups seized large parts of Marawi, in Mindanao, to establish a "province" of the Islamic State in Iraq and Syria.

Mr Duterte told reporters that any effort to talk to the militants was already too late.

"If they go to the Maute to talk, talk about what? Surrender? Settlement? Just like that?... I will not (negotiate). How about my soldiers who have died? How about (the terrorists') murderous rampage?" he said.

Security officials have lately been arresting suspected militants trying to flee the fighting in Marawi.

A purported sister of brothers Omarkhayam and Abdullah Maute, who started the Maute group and led the attacks on Marawi, and two other suspected militants were arrested on board a docked ferry.

The trio had been heading to the capital Manila.

Yesterday, security officials reported that they had seized 11kg of methamphetamine, known locally as "shabu", on Sunday night from retreating Muslim militants in Marawi.

The drugs are estimated to have a street value of up to 250 million pesos (S$6.9 million).