MANILA – After the military said last week it was looking into reports that senior Abu Sayyaf leader Isnilon Hapilon had already left Marawi City, Defense Secretary Delfin Lorenzana on Monday said one of the world’s most wanted terrorists remains holed up in the besieged city.

"According to our latest information, he is still inside Marawi. He is hiding inside one of the mosques in Marawi. I think this is maybe correct,” Lorenzana said in a news briefing in Malacañang.

The defense chief said Hapilon has not returned to his home island of Basilan, according to information from the military's civilian assets.

"There were three fighters from Marawi that arrived in Basilan more than a week ago, but Isnilon was not one of them. So we still believe that he is still in Marawi," he said.

Hapilon has been designated as the point person in Southeast Asia of the Islamic State in Iraq and Syria (ISIS). He and his men backed the Maute group, led by brothers Abdullah and Omar, in laying siege to Marawi City in a supposed bid to establish an ISIS province in Mindanao under a caliphate in the region.

Clashes erupted in the city on May 23 after government troops attempted to arrest Hapilon, who carries a $5 million bounty on his head being offered by the US government. The Philippine government and military said while government troops failed to catch Hapilon, the operation thwarted a bigger attack by the terrorists.

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At least 459 people - 336 suspected terrorists, 39 civilians, and 84 government troops - have died since the clashes erupted.

Marawi, the most important Muslim city in the mainly Catholic Philippines, has also been left in ruins after more than a month of fighting.

Some 400,000 civilians from Marawi and outlying areas have also been displaced as a result of the fighting.

While government has put the civilian death toll at 39, the military believes this could “increase significantly” as troops have yet to reach other parts of the city where some trapped civilians were feared to have been executed.

The clashes prompted President Rodrigo Duterte to place the entire Mindanao under martial law, citing the militants’ bid to establish an Islamic State province in Mindanao. Government, nonetheless, insists there was no failure of intelligence that led to the crisis, saying the military was able to preempt a much larger attack by the militants.

• Duterte: Marawi crisis not a failure of intelligence

The emergence of groups pledging allegiance to the terror group has been considered as the biggest security problem to face the year-old Duterte administration.

The rise of pro-ISIS groups in the country has also raised alarm in Washington and the Philippines’ neighbors in the region, which fear that the notorious terror group was seeking to establish a new front in Asia amid its successive losses in Iraq and Syria.

While the military has expressed confidence that the crisis will be over soon, security experts worry that the Marawi attack was just part of a bigger plan of Islamic State in Southeast Asia.

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