President Rodrigo Duterte said on Sunday that he has accepted the offer of Nur Misuari, founding chairman of the former secessionist Moro National Liberation Front, to allow 2,000 Muslim fighters to join the government forces in going after the terrorists in Mindanao.

In an interview in Subic Bay where he toured a Japanese warship, Duterte said he had a talk with MNLF founding chairman Nur Misuari on Saturday night.

"He (Misuari) has offered to ask his troops of the MNLF, able bodied ones still, to fight with government and I accepted the offer...he has offered 2,000 and I said I'm willing to take in 2,000 as regular troops of the Armed Forces of the government," he said.

He said Misuari initially offered 3,000. But he said the government could accommodate 2,000 at the moment.

The government troopers have been pursuing the Islamic State-linked Maute Terror Group in Marawi City.

The rebel Moro Islamic Liberation Front has also been helping the government in rescuing the trapped residents in Marawi.

The government forged a final peace agreement with the MNLF in 1996, and a Comprehensive Agreement on the Bangsamoro with the MILF in 2014.

Duterte said if the government was not caring for the civilians who could be killed, the military could easily finish the Maute problem in 24 hours by just dropping bombs in Marawi.

"I can end this war in 24 hours. All I have to do is to order the Air Force to release all their bombs and their arsenal and their ordnance. And I will level everything down there on the ground," he said.

However, the government could not do that, he said.

"If we kill a lot of civilians, more than the rebels, we lose the fight because we lose the moral ascendancy, because it seems that we do not care about lives, about human rights and all," he said.

The government said that least 20 civilians have already been killed by the terrorists since they started occupying Marawi on May 23. (Celerina Monte/DMS)