The Philippines government said foreign fighters (including Malaysians and Indonesians) have joined an Islamic State-linked militant group occupying a southern Philippine city.

The authorities said they believe the group – which is holding high ground while the military is slowed down on lower grounds – are seeking to turn the island of Mindanao into a caliphate.

The latest news on the conflict indicates that Philippines troops fired mortars in Marawi City as they battled Islamist militants of the Maute group who were holding hostages and were reported to have murdered at least 11 civilians.

The Philippines is calling the development an invasion, which has seen wider call for support to its President Rodrigo Duterte.

The source that spoke to the Wall Street Journal said while the authorities had long known foreign fighters were on the side of militant groups in lawless parts of Mindanao, this attack is an escalation of a fragile situation.

The current fight has seen casualties among militants, local and foreign, soldiers and police officers.

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Duterte on Tuesday declared martial law for the entirety of Mindanao—home to 22 million people.

The government said the events in Mindanao is no longer a rebellion of Filipino citizens.

It has transformed into invasion by foreign terrorists who heeded the clarion call of ISIS to go to the Philippines if they feel difficulty in going to Iraq or Syria.

The Malaysian Deputy Prime Minister Datuk Seri Ahmad Zahid Hamidi confirmed reports that two Malaysian men were killed in clashes in Mindanao.

He said he had been informed of their identities and background but will leave it to the police to release further information.

The Philippine Abu Sayyaf militants are said to have links with Syria-based global terror network Islamic State.

Malaysia’s Special Branch police said its intelligence indicates at least five other Malaysians are still in Marawi City and that it will take the time to verify the numbers and their identities, said The Malay Mail Online.