The Philippine military said on Thursday there was a “big possibility” that a top Malaysian militant tipped to become ISIS’ point man in Southeast Asia has been killed in an overnight battle.

Thirteen rebels among the few dozen ISIS loyalists holed up in the decimated heart of Marawi City were killed in the operation and Malaysian Mahmud Ahmad was likely among them,taskforce deputy commander Colonel Romeo Brawner said.

“There is a big possibility that Dr Mahmud is among them but we will only be definite once we have a match of probably DNA samples, maybe of the dental records,” Brawner told reporters.

If confirmed, the death of Mahmud would be a significant blow to any effort by ISIS, which is on the back foot in Syria and Iraq, to establish a presence in Mindanao, a vast Philippine island where lawlessness, poverty and rebellion have prevailed for decades.

Mahmud, one of Malaysia's most wanted men, is believed to have been pivotal in funding the siege of Marawi, which has lasted almost five months and killed more than 1,000 people, mostly rebels.

Some experts say he could become ISIS’ “emir” in Southeast Asia after the death on Monday of Isnilon Hapilon, the head of the militant alliance that laid siege to Marawi.

Hapilon, a faction leader of the Abu Sayyaf group and wanted by the US Federal Bureau of Investigation, was killed early on Monday along with Omarkhayam Maute, one of two leaders of the Maute militant clan, with whom he teamed up with to try to carve out an ISIS “Wilaya” in the southern Philippines.

The alliance was strengthened by fighters from Indonesia, Malaysia, Singapore and the Middle East, among other countries, and Mahmud, a 39-year-old former university lecturer, is believed to have been central to attracting funds to finance the operations.

He was seen in a video alongside Hapilon and the Maute brothers plotting the Marawi siege. Security experts say he studied in Pakistan before going to Afghanistan where he learned to make bombs at an al Qaeda camp. He left Malaysia in 2014.

Brawner reiterated that the military was not certain of Mahmud's death and was still trying to determine how many fighters were left.

“The resistance is still there. In fact, we can hear from the background, the battle is ongoing,” he said, as intermittent gunfire rang out.