MANILA — Philippine President Rodrigo Duterte warned today that martial law would be "harsh" and like a dictatorship, after imposing military rule in the south of the country to combat Islamist militants.

Duterte declared martial law in the southern region of Mindanao, home to about 20 million people, on Tuesday night after gunmen who had declared allegiance to the Islamic State group rampaged through a southern city.

At least three security force personnel died on Tuesday as they battled the militants in Marawi, a mainly Muslim city of about 200,000 people, with authorities reporting that gunmen burned a Catholic Church and other buildings.

"Our fellow Filipinos, do not be too scared," Duterte said from Moscow, where he had just begun an official visit that he abruptly ended to fly home and deal with the crisis.

Duterte vowed to be ruthless in quelling the terrorism threat in Mindanao, drawing parallels with martial law imposed by former president Ferdinand Marcos during his two-decade rule that ended with a "People Power" revolution in 1986.

"It could not be any different from what President Marcos did," Duterte said, as he reminded Filipinos of his election campaign pledge last year to be "harsh" in dealing with terrorism.

"What I told everyone, do not force my hand into it. I have to do it to preserve the Republic of the Philippines, the Filipino people," he said.

Duterte also said that martial law in Mindanao could last up to a year.

Critics of Duterte, who has waged a controversial war on drugs that has claimed thousands of lives, have feared that he may use various security threats as an excuse to impose a form of authoritarian rule.

Opposition Senator Francis Pangilinan, president of the Liberal Party, expressed concern on Wednesday that martial law could lead to military abuses, citing extrajudicial killings under Marcos.

"Our painful experience with the imposition of martial law under the Marcos dictatorship should serve as a reminder that we must, as citizens, stay vigilant," Pangilinan said in a statement.

Deadly clashes

The fighting in Marawi erupted on Tuesday afternoon after security forces raided a house where they believed Isnilon Hapilon, a leader of the infamous Abu Sayyaf kidnap gang and Philippine head of IS, was hiding.

The US regards Hapilon as one of the world’s most dangerous terrorists, offering a bounty of US$5 million for his capture.

More than 100 gunmen responded to the raid by burning buildings and conducting other diversionary tactics, according to Defence Secretary Delfin Lorenzana.

Photos posted on social media by residents showed the gunmen walking through the streets of Marawi and placing black flags that looked similar to those used by IS.

Lorenzana said on Tuesday night that many gunmen were hiding in buildings as snipers, making it difficult for security forces to combat them.

There were no reports of fighting on Wednesday morning, although it was unclear if the militants were still in the city or had escaped into nearby mountains and forests that they have long used as hideouts.

Authorities did not give any updates on the whereabouts of Hapilon.

The Abu Sayyaf, based on the most southern islands of Mindanao, has kidnapped hundreds of Filipinos and foreigners since the early 1990s to extract ransoms. The US lists it as a terrorist organisation.

Security analysts say Hapilon has been trying to unite Filipino militant groups that have professed allegiance to IS.

These include the Maute group, which is based near Marawi.

The Maute group has engaged in repeated deadly battles with the military over the past year.

Muslim rebels have been waging a rebellion since the 1970s for an independent or autonomous homeland in Mindanao, with the conflict claiming more than 130,000 lives.

The main Muslim rebel groups are involved in peace talks with the government.

But the Abu Sayyaf, Maute and other hardline groups are claiming they want to set up an Islamic caliphate in the south for IS, according to security analysts.

The US and other Western governments warned this month that terrorists were planning to kidnap foreigners in tourist hotspots in the western and central Philippines, adding to longstanding advisories of abduction threats in Mindanao. — AFP