Local resident Chico Usman said the militants had entered the predominantly Muslim city of some 200,000 suddenly, on the afternoon of May 24, wearing masks and carrying assault rifles. "Everybody was shocked and ran into their houses," he said, adding they could hear gunfire and fighting until the following morning.
Black ISIS flags emblazoned in white with the words "There is no god but God" were flying from "every corner in the city," said Usman, who spoke to CNN from near Saguiaran, a town outside Marawi, where thousands of fleeing residents had taken temporary shelter.
The latest violence flared up on May 23, after the military launched an operation targeting Isnilon Hapilon, a Filipino militant leader, who was last year designated ISIS emir for Southeast Asia.Â
Surrounded and fearing capture, Hapilon is thought to have issued an emergency call for reinforcements from members of the Maute group, a local Islamist militant organization that's pledged allegiance to ISIS, who poured into Marawi by the hundreds, setting fire to buildings, taking hostages and entering into running street-battles with government forces as they came.
While Islamist and criminal groups have been active in the lawless tri-border area between the Philippines, Malaysia and Indonesia for years, such an audacious and aggressive attack on government troops by fighters loyal to ISIS has shocked many observers -- and increased fears the group is succeeding in extending its influence into Southeast Asia.
According to Otso Iho, a senior analyst at Jane's Terrorism and Insurgency Center, increased cooperation between groups is a significant step towards "generating a much more unified Islamist front particularly in the southern Philippines."
A report by the Jakarta-based Institute for Policy Analysis of Conflict (IPAC), however, noted that while ISIS "has deepened cooperation among extremist groups in Southeast Asia," law enforcement and counter terrorist efforts remain largely national.
If action is not taken, they said, the region risks becoming a Southeast Asian version of the tribal areas along the border of Afghanistan and Pakistan.
Risks to which Mindanao is particularly vulnerable.
The southernmost island in the Philippines, Mindanao has long been plagued by conflict, with the region experiencing Communist insurgencies, nationalist rebellions, and brutal military crackdowns launched from Manila.Â
According to the United Nations World Food Program, outbreaks of conflict in 2000 and 2008 "each led to the displacement of nearly a million individuals."
In a 2016 video circulated online, armed men -- most appearing no older than teenagers -- stand holding assault rifles and other weapons, as Arabic music plays in the background. They join their hands and pledge allegiance to Abu Bakr al-Baghdadi, the self-declared caliph of the Islamic State.Â
Later in the video, a Malaysian fighter urges viewers who cannot travel to the Middle East to go instead "to the Philippines." He then joins two other fighters -- identified as Filipino and Indonesian -- in beheading three Christian captives.Â
Bearing the trademark high production values and gruesome executions of the core ISIS propaganda wing, the video represented the growing importance of Southeast Asia to the group's leaders in Syria. Recent issues of Rumiyah -- a monthly propaganda magazine published by ISIS in multiple languages -- have also highlighted actions by Islamic State fighters against the "Filipino Crusader army" in the region.Â
He was endorsed by ISIS as the emir for Southeast Asia in 2016, according to IPAC despite the fact he speaks "neither Arabic nor English, and his religious knowledge is limited."
Regional cooperation in combating militants such as Hapilon may become more important than ever before, as experts predict loss of land in Syria and Iraq will see ISIS leadership turn more attention to the southern Philippines.
Whether because they are sent there by al Baghdadi, or because they are fleeing Syrians and Kurds and other fronts, there are hundreds of local Southeast Asian fighters poised to bring their expertise and ideological commitment to the region.
Australian Attorney General George Brandis who is a strong proponent of a more joined-up approach -- and who recently labeled the security threat of returning fighters as among the "greatest in the region" -- has said the matter would be discussed at an upcoming meeting of Asian security ministers later this year.Â
"Governments in the region have to constantly be vigilant against the threat of the returnees," said Singapore-based analyst Bin Jani.
Along with the hundreds of fighters in the Middle East known to authorities, there are many more who are not being tracked, Bin Jani said, "militants can be very unassuming when they're not carrying their guns."
CNN's Steve George contributed to this report.