By Neil Jerome Morales and Tom Allard

MARAWI CITY, Philippines (Reuters) – Islamist militants who seized the Philippines town of Marawi two weeks ago have stockpiled weapons and food in mosques, tunnels and basements to prepare for a long siege, officials said on Monday.

Among the several hundred militants linked to the Islamic State group are fighters from Indonesia, Malaysia, India, Saudi Arabia, Chechnya and Morocco.

The battle for Marawi City has raised concerns that the ultra-radical jihadist group is building a regional base on the island of Mindanao, at the southern end of the Philippines.

Parrying questions on why the fighters had been able to resist the Philippine army for so long, senior officers said the main problem was that 500-600 civilians were still trapped in the urban heart of the town.

President Rodrigo Duterte said on Saturday that Marawi City would be fully liberated within three days, but on Monday officials were more circumspect and gave conflicting estimates of how many combatants were holding out.

Major General Carlito Galvez, head of the military command in Western Mindanao region, said as many as 200 fighters from the Maute militant group and others were still inside the town, and had prepared in advance for a long standoff.

“… the Maute, even if they fight two months, they will not starve here,” he told a news conference about a kilometer from the fighting.

“There are underground tunnels and basements that even a 500-pounder cannot destroy.”

He said that, days before seizing the city of 200,000 people, the militants had placed supplies in mosques and madrasas, or Islamic religious schools. Although the Philippines is largely Christian, Marawi City is overwhelmingly Muslim.

Fighting had erupted on May 23 after a bungled raid aimed at capturing Isnilon Hapilon, whom Islamic State proclaimed as its “emir” of Southeast Asia last year after he pledged allegiance to the group. The U.S. State Department has offered a bounty of up to $5 million for his arrest.

The military said on Monday that Duterte had offered a bounty of 10 million pesos ($200,000) to anyone who “neutralized” Hapilon, and 5 million pesos for each of the two leaders of the Maute group.

CIVILIANS TRAPPED

Brigadier General Restituto Padilla told a news conference that the militants now held less than 10 percent of the city, but that meeting Duterte’s deadline was not easy.

“Complications have been coming up: the continued use of civilians, potential hostages that may still be in their hands, the use of places of worship … and other factors that complicate the battle because of its urban terrain,” he said.

Reuters correspondents saw military helicopters flying combat sorties over Marawi City and smoke rising from parts of town amid machinegun fire.

A four-hour ceasefire to evacuate residents was marred by gunfire on Sunday, leaving hundreds of civilians stuck in their homes.

Padilla said that 1,467 civilians had been rescued so far, and that the 500-600 still trapped were low on food and water.

“There are places that we use as passageways to enemy territory – when we reach those areas, sometimes we see old people who are weak, cannot move on their own, because of lack of food,” he said.

A presidential spokesman said 120 militants had died in the battle, along with 38 security personnel. The authorities have put the civilian death toll at between 20 and 38.

President Duterte, who launched a ruthless ‘war on drugs’ after coming to power a year ago, has said that the Marawi fighters were financed by drug lords in Mindanao, an island the size of South Korea that has suffered for decades from banditry and insurgencies.

After the Marawi siege began, Duterte declared martial law in Mindanao, with support from allies in the Congress. On Monday, six opposition lawmakers challenged the move in a petition to the Supreme Court. ($1 = 49.4050 Philippine pesos)

(Additional reporting by Karen Lema in MANILA; Writing by John Chalmers; Editing by Kevin Liffey)