President Rodrigo Duterte joked on Friday that soldiers could rape up to three women, as he reassured them of his full support under his newly declared martial law in southern Philippines.

Duterte made the comments in jest during a speech to soldiers on Mindanao island, where he imposed military rule law on Tuesday to try to crush ISIL-linked fighters, who have been battling the army after laying siege to Marawi City.

The president, who is known for remarks often deemed offensive, reiterated that only he would be liable for any backlash. He has, however, said he would not tolerate abuses.

"If you go down, I go down. But for this martial law and the consequences of martial law and the ramifications of martial law, I and I alone would be responsible. Just do your job, I will take care of the rest," Duterte said at an army base in Iligan City, outside Marawi City, according to a president's office transcript.

"I'll imprison you myself," he said, referring to any soldiers who commit violations. Then he joked: "If you had raped three, I will admit it, that's on me."

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This was not the first time Duterte has made a joke about rape.

In the lead-up to his presidential election win last year, he caused an outrage when he recalled a 1989 prison riot in which an Australian missionary was killed, and inmates had lined up to rape her.

In what was intended as a joke, Duterte said the victim was "beautiful" and as mayor of Davao City, where the riot took place, he should have been first in line. He later apologised and said he did not intend to disrespect women or rape victims.

Duterte is known for his informal style and his speeches are often loaded with profanity, threats and jokes about taboo subjects, which offend some, but are taken lightly by many Filipinos.

More bombs

Meanwhile, Philippine security forces on Saturday dropped more bombs on Marawi, as truckloads of troops were driven into the city.

"We have identified where they are consolidating so we are doing surgical air strikes to destroy the local terrorist group," military spokesman Lieutenant-Colonel Jo-ar Herrera told the AFP news agency.

Marawi is one of the biggest Muslim-populated cities in the mainly Catholic Philippines. Its population of about 200,000 residents had largely been evacuated because of the fighting.

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The violence erupted on Tuesday when dozens of gunmen went on a rampage throughout Marawi City in response to an attempt by security forces to arrest Isnilon Hapilon, a veteran Filipino fighter regarded as the local leader of armed groups that have pledged allegiance to ISIL.

The gunmen planted black ISIL flags, took a priest and up to 14 other people hostage from a church, and set fire to buildings.

Duterte has vowed to extinguish the threat of the fighters, whom he has said belong to the banned local Maute group but are being backed by criminals in the area.

Armed fighters working with criminals and corrupt politicians are common across Mindanao, where a Muslim separatist rebellion has claimed more than 120,000 lives since the 1970s.