"I'd be harsh", Duterte said in an interview with his assistant communications secretary while onboard a flight back to Manila.
The scene of the firefight was near police headquarters and the campus of Mindanao State University, officials said.
On why the fighting has dragged on, Lt-Gen Galvez said: "We have to understand the danger of urban fighting.
Martial law and the heightened vulnerability to military abuse that it brings to women and children are not a joke either." r Duterte's aides frequently explain away his most controversial comments as being "merely rhetoric" or comments which only ordinary Filipinos would appreciate.
"The main objective of the offensive is to suppress the lawlessness and to maintain normalcy in Marawi so that our people here, our countrymen, can return, especially by Ramadan", he said, adding that he hopes the city would be cleared within three days.
Two soldiers and a policeman were killed and 12 people wounded amid chaos in Marawi, a predominantly Muslim city of about 200,000 people, where members of the Maute militant group took control of buildings and set fire to a school, a church and a detention facility.
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"The Philippines is facing a risky group with more solid global connections", security expert Rommel Banlaoi said of the Maute.
Duterte was visiting soldiers at a a military base near Marawi, where fighting has broken out between Maute terrorists who have pledged their allegiance to Daesh and government forces, when he made his controversial remarks.
"Some have been here a long time, some of whom have been here helping, teaching, aiding, connecting them and linking these groups", said Padilla.
In a sign of the confusion over events inside the city, a local police chief told The Associated Press on Friday that he was alive and well - two days after President Rodrigo Duterte told the media he had been beheaded by militants.
"We will conduct house-to-house clearing and do everything to remove the threat there".
"They did some burnings, they showed up in another area so it looked chaotic, but it's actually a small group facing an overwhelming number of government forces", Ano said.
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Ano also said foreign fighters were believed to be inside, but he was more cautious.
The military on Saturday said no civilians have been confirmed killed in air strikes targeting members of the terrorist Maute and Abu Sayyaf groups in Marawi City.
The operation comes as the city enters the fifth day of a siege by government forces.
Military intelligence said the Maute brothers also had links to two of the region's most unsafe militants - Indonesian Ustadz Sanusi and Malaysian bomb maker Zulkifli bin Hir, alias Marwan.
THE Philippines Commission on Human Rights has cautioned the military and national police against carrying out arbitrary arrests amid the government's implementation of martial law in restive Mindanao.
Government troops have been intensifying offensives on the island strongholds of Abu Sayyaf and last month killed about 36 Maute group fighters when aircraft bombed their mountain lair in Lanao del Sur province.
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