The US has drafted in special forces to assist the Philippines Army in an advisory capacity as the battle intensifies to regain control of the southern city of Marawi, under occupation by Islamic State-affiliated militants.

The Philippines military suffered heavy losses in a 14-hour gun battle on Friday in the city that killed 13 soldiers and left 40 wounded in “intense” close-quarter combat.

“They are not fighting. They are just providing technical support,” Joint Task Force Marawi spokesperson Lieutenant Colonel Jo-Ar Herrera told a news conference in Marawi City. A US P3 Orion surveillance plane is now operating in the skies above the city, according to local media.

The move is surprising given the strained relationship between the US and the Philippines in recent months, but highlights the scale of the task faced by government forces to end the Islamist insurgency on the island of Mindinao.

Philippines marines targeted Maute insurgent positions at 3:30am Friday morning in a battle that lasted until 5pm. The Islamic State (IS, formerly ISIS/ISIL)-affiliated fighters used improvised explosive devices (IEDs) and rocket-propelled grenades to repel the assault.

Herrera said the battle was a “temporary setback [that] has not diminished our resolve a bit.”

He also described the difficulties the Philippines military has faced in ending the Maute insurgency in the city.

“The key component we saw in the firefight with the marines was how the terrorists are using mosques. The enemies took defensive positions there. They also used civilians as human shields,” Herrera said, as cited by Rappler.

Herrera added that the military is investigating reports that the founders of the Maute group have been killed in the fighting.

“We are still awaiting confirmation. We are still validating those reports but there are strong indications,” he said.

Manila previously announced plans to “liberate” the city by Monday, June 12 the Philippine Independence Day, but according to an evacuated Marawi City resident in direct contact with RT, the insurgents have received reinforcements in recent days and have further entrenched their positions.

“They [the military] can’t handle the situation anymore. Instead of going forward, now they are going back because the terrorist group got reinforcements. The situation now is more complicated,” Chico Dimaro Usman told RT.

Friday’s battle is the biggest one-day loss experienced by the Philippines military since a friendly-fire incident on June 1 which killed 10 soldiers.

So far, 58 members of the Philippines security forces, 158 Maute fighters and 21 civilians have been killed in the fighting. Some 1,599 civilians were rescued as part of the military operation, according to Brigadier General Restituto Padilla, as cited by ABS-CBN.

The Maute group reportedly joined forces with Isnilon Hapilon, one of the leaders of the Abu Sayyaf jihadist group, who IS proclaimed as its Southeast Asian emir.

Military officials believe Hapilon is still in Marawi and President Rodrigo Duterte has offered a P10 million ($200,000) bounty for Hapilon’s “neutralization.” Hapilon features on the FBI’s ‘most-wanted terrorist’ list and currently has a $5 million bounty on his head.

The US previously deployed special forces soldiers to Mindanao in 2002 to advise Philippines units fighting Abu Sayyaf militants in a program that once involved 1,200 Americans, but was discontinued in 2015.