The Philippines military deployed helicopters and special forces Friday amid fierce battles with Islamic State group-affiliated jihadists who have besieged the southern city of Marawi, in what one leading terror expert says is part of a bid to establish an ISIS province.

Clashes continued for the fourth day Friday, as government forces carried out air strikes in a bid to expel Maute clan militants from their positions in the city. Officials say more than 40 people were killed as militants rampaged through Marawi, taking local Christians as human shields, torching a church and government buildings, freeing prisoners from jail, and sparking a mass exodus of terrified residents.

“ISIS is shrinking in its battle space, in its heartland, but it’s decentralizing,” Rohan Gunaratna, head of Singapore’s International Centre for Political Violence and Terrorism Research, told VICE News. “This is the new structure of global jihad 3.0 – we will have the global expansion of ISIS.”

At least six foreign fighters from Indonesia and Malaysia have been found among the 31 rebels killed so far, further evidence that jihadists heeded a call from ISIS to join affiliated groups in the Philippines if they were unable to travel to Iraq or Syria.

Solicitor General Jose Calida told reporters Friday that what was happening in the city was “no longer a rebellion of Filipino citizens.”

“It has transmogrified into invasion by foreign terrorists, who heeded the call of the ISIS to go to the Philippines if they find difficulty in going to Iraq and Syria,” he said. “They want to make Mindanao part of the caliphate.”

In response, Philippines President Rodrigo Duterte has declared martial law over the entire southern island of Mindanao, where Marawi is located, and warned it could be applied to the rest of the country in a bid to stop the spread of radical Islam. The Philippines is largely Catholic, but the southern third of the country is home to a sizable Muslim minority and the government has battled for decades with multiple rebel groups fighting for an separate Islamic state in the south.

The crisis was sparked Tuesday when the army raided the local hideout of Isnilon Hapilon, the leader of ISIS-affiliated militant group Abu Sayyaf who has a $5 million U.S. State Department bounty on his head. In response, Hapilon called for the support of the Maute clan, an allied Islamist militia active in the region, whose gunmen rampaged through the town.

Gunaratna said the Maute clan, led by brothers Abdullah and Omar Maute, had about 300 fighters and was the strongest of four allied militant groups who had pledged allegiance to ISIS – about 1000 fighters in total. The Maute were blamed for a failed bombing in November near the U.S. embassy in Manila.

Hapilon, the leader of the Abu Sayyaf Group, had been appointed emir of all ISIS-affiliated forces in the Philippines by ISIS leader Abu Bakr al-Baghdadi last year, Gunaratna said. ISIS claimed responsibility Wednesday for the actions in Marawi via its Amaq news agency.

Gunaratna said the groups, previously al Qaeda-affiliated, had drifted towards ISIS’s more violent ideology in recent years. “Now they have become more barbaric and brutal like ISIS, they think that by doing this they can achieve their goals.” he said.

“It’s like violent pornography – they have highly visual violence”

Through the attack on Marawi, Gunaratna says the groups hope to demonstrate that they could seize and hold cities, and thus graduate from being an “emerging vilayat,” or region, of the caliphate, to becoming a fully declared province. “That’s why the Marawi fight is so important for them,” he said.

Officials are working to establish safe zones for Marawi residents left in the city, and supply them with provisions, as local shops remain shuttered amid the fighting. Other parts of Mindanao, including Duterte’s home city of Davao, are under high alert as authorities fear the militants could launch attacks. Jo-Ar Herrera, spokesman for the First Infantry Regiment told reporters that the military believed Hapilon was still at large in the city.

According to Gunaratna, the Philippines has become an increasingly critical training base for jihadists from around the world in recent years. A Belizean bombmaker was killed by government troops in January, and a Moroccan terrorist died in a firefight with the military in April last year. Gunaratna said Duterte’s concern about the growth of ISIS in the southern Philippines was warranted, but that martial law was unlikely to solve the problem. What the Philippines government needed was better equipment and training for its forces to effectively tackle ISIS, and closer intelligence-sharing with Western governments.