PETALING JAYA - Twenty-eight Malaysians have joined the ISIS-backed Maute militants in the battle against Philippine forces in Marawi on the southern island of Mindanao, the Malay Mail reported on Sunday (May 28), citing a Manila-based intelligence official.

"Intelligence from ground operatives disclosed 28 Malaysians arrived in Marawi early last week for a religious event. We suspect there must be more who arrived separately," the official was quoted as saying.

"Initially, nobody knew what happened to them ... but after the deaths of two Malaysians in firefights with the armed forces, we can conclude they too took up arms."

There are also those from the Middle East in Marawi City or on the outskirts, the official said.

Fighting between Philippine government forces and the Muslim militants in Marawi is now into its sixth day. The Philippine military has said foreign militant fighters including Malaysians, Indonesians and Singaporeans are involved in the clashes, joining what the government says is a bid "to establish an Islamic state (in Mindanao)".

Maute gunmen had seized large parts of Marawi on Tuesday as they thwarted the government's attempt to capture Isnilon Hapilon, the most senior leader of the Islamic State in Iraq and Syria (ISIS) in South-east Asia.

The US has offered a US$5 million (S$6.9 million) reward for his capture.

The government underestimated the militants protecting Hapilon, and the raid went awry.

The fighting led President Rodrigo Duterte to declare martial law for 60 days for the entire southern island group of Mindanao, where Marawi is located.

About four-fifths of Marawi's population of about 200,000 have been evacuated so far.

Government helicopter gunships on Saturday used guided rockets for the first time against the militants, as truckloads of marines were seen driving into Marawi.

Security forces also dropped more bombs yesterday on targets inside the city and in the outskirts.