DAVAO CITY - Just when it has been written off as a spent force, the Maute group of Islamic militants has staged its biggest act yet - seizing parts of a city earlier this week and continuing to hold off government troops as of Thursday (May 25) evening.

More than 100 Maute guerillas laid siege on Tuesday to Marawi, a Muslim city in the largely Catholic country, leading President Rodrigo Duterte to declare martial law in the southern island group of Mindanao where the city is located.

The guerillas had seized the city's main road and two bridges, torched buildings and held several hostages. They were seen in photos parading around the city on vehicles brandishing the black flag of the Islamic State in Iraq and Syria (ISIS).

The militants held on as of Thursday evening despite battling an Army brigade, backed by armoured vehicles and helicopters.

Tactically smart and adept at using social media, the Maute is based in Lanao del Sur, the Mindanao province which includes Marawi.

The group represents a new brand of extremism in the Philippines, as it is driven more by ideology than commerce or a separatist agenda, making it more lethal and resilient than other groups.

In an October 2016 report, regional security expert Sidney Jones said the Maute "has the smartest, best-educated and most sophisticated members of all of the pro-ISIS groups in the Philippines".

Maute was named after two brothers who studied at Islamic schools while working in the Middle East.

Omarkhayam Maute, who studied at the Al-Azhar University in Cairo, married the daughter of a conservative Indonesian Islamic cleric. His brother Abdullah studied in Jordan.

A diplomat who met Omarkhayam in Cairo said he was "like any other Filipino Muslim".

"He seemed to me like the typical devout Muslim, judging by what he was wearing. He had a turban and a goatee… I wouldn't say at the time that he came across as a radical," he told The Straits Times.

The Maute, which had surfaced around 2013 and pledged allegiance to ISIS last year, has raised its profile since Mr Duterte took officenearly a year ago.

The group was blamed for bombing a street market in the president's hometown of Davao City in September, killing 15 people.

A foiled attack near the United States embassy in Manila in November revealed that it already had cells operating north of the capital Manila.

In December, following another bomb attack, Mr Duterte accused the Maute of dealing in drugs.

"Maute is rampaging now… They want money for their uprising. To sustain it, they're dealing in drugs."

In April this year, government troops seized a Maute camp said to be big enough to house at least 200 fighters, with trenches, bunkers, and bomb-making implements. Military officials had declared then that the group was in its death throes.

But it is now clear that they had spoken too soon.

Analysts say what is worrying is that the Abu Sayyaf, another ISIS-linked group, appears to be cementing its ties with the Maute.

The military's recent offensives against the Maute were driven by reports that Isnilon Hapilon, the Abu Sayyaf's chief and ISIS's top man in South-east Asia, has moved from the Sulu archipelago to join forces with the Maute to establish an ISIS province in central Mindanao.

Intelligence reports said dozens of battle-hardened ISIS militants from Iraq and Syria are now fighting with the Maute in Mindanao.

"The Philippines is facing a dangerous group with more solid international connections," security expert Rommel Banlaoi told Reuters.

Calling this "a game changer", he added: "We haven't seen anything like this before."