US special forces are helping the Armed Forces of the Philippines (AFP) to end a siege of the southern town of Marawi by militants allied to Islamic State, a US embassy spokesperson in Manila says.

Key points: US troops will not fight, but provide technical support, Philippines military say.

US troops will not fight, but provide technical support, Philippines military say. Battle for the city of Marawi currently in its third week as thousands flee.

Battle for the city of Marawi currently in its third week as thousands flee. Death toll now stands at 58 marines, 20 civilians, and more than a hundred rebels.

The seizure of Marawi on May 23 by hundreds of fighters who have sworn allegiance to Islamic State, including dozens from neighbouring countries and the Middle East, has fuelled concern that the ultra-radical group is gaining a foothold in South-East Asia.

"At the request of the government of the Philippines, US special operations forces are assisting the AFP with ongoing operations in Marawi that help AFP commanders on the ground in their fight against Maute and ASG militants," the spokesperson said.

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ASG stands for the Abu Sayyaf militant group.

The US spokesperson gave no details of the US involvement.

At least 200 militants are holed up in a corner of the town with an estimated 500 to 1,000 civilians trapped there.

Philippine military spokesman Lieutenant Colonel Jo-Ar Herrera confirmed the US special forces would be assisting local troops to end the siege, but only to provide technical support.

"They are not fighting. They are just providing technical support," Lieutenant Colonel Jo-Ar Herrera said.

Until now there had been no confirmation that the Philippines had sought US assistance in the battle for Marawi City on the island of Mindanao, which is in its third week.

Thousands of Marawi's 200,000 inhabitants fled the city after Islamic State-linked militants executed civilians, while the nation battled a deadly casino attack last week labelled as a 'terrorist' act by a top Philippine lawmaker.

About 40 foreigners have fought alongside the Philippine militants, most of them from Indonesia and Malaysia, though some came from the Middle East.

The seizure of Marawi has alarmed South-East Asian nations which fear Islamic State — facing setbacks in Syria and Iraq — is establishing a stronghold on the Philippine island that could threaten the whole region.

It's unclear how many people remain trapped in Marawi as government troops battle Muslim militants led by the so-called "Maute" group. ( AP: Aaron Favila )

Death toll rises

The Philippines military suffered its biggest one-day loss since 10 troops were killed in a friendly-fire incident on June 1.

Lieutenant Colonel Jo-Ar Herrera announced there had been 13 Philippine marines killed in the battle, a setback in the government's quest to retake the city.

Who are the Maute? The Maute are an armed Muslim group that has pledged allegiance to the Islamic State group.

The Maute are an armed Muslim group that has pledged allegiance to the Islamic State group. Hapilon was reportedly designated the leader of the alliance.

Hapilon was reportedly designated the leader of the alliance. The Maute has been blamed for a bomb attack that killed 15 people in southern Davao city, Mr Duterte's hometown, last September.

The Maute has been blamed for a bomb attack that killed 15 people in southern Davao city, Mr Duterte's hometown, last September. In April, troops killed dozens of Maute militants and captured their jungle camp near Lanao del Sur's Piagapo town.

In April, troops killed dozens of Maute militants and captured their jungle camp near Lanao del Sur's Piagapo town. Troops found homemade bombs, grenades, combat uniforms and passports of suspected Indonesian militants in the camp, the military said.

He said the marines were conducting clearing operations died after an "intense" house-to-house firefight during which they encountered improvised explosive devices and were attacked by rocket-propelled grenades.

The deaths take the number of security force members killed to 58, with 20 civilians and more than a hundred rebel fighters also killed in the Marawi battles.

He said the Philippines military are validating reports that two brothers leading the Maute Islamist militant group were killed.

"We are still validating those reports but there are strong indications," he said.

Mosques, civilians remain priority

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Some are being held as human shields, while others are hiding in their homes with no access to running water, electricity or food.

The military said it was making headway in the town but was proceeding carefully so as not to destroy mosques where some of the militants had taken up positions.

"We give premium to the mosques, because this is very symbolic to our Muslim brothers," Lieutenant Colonel Jo-Ar Herrera said.

The Philippines is majority Christian, but Mindanao has a significant population of Muslims and Marawi City is overwhelmingly Muslim.

The Philippine military has said the aim was to end the siege by Monday, the Philippines' independence day.

"As long as we sustain the fight, as long as we are able to destroy the elements of the local terrorist group we can reach that," he said of the June 12 target.

The little-known Maute group has been a fierce enemy of a military with superior firepower and greater troop strength.

The Philippine military said 13 marines have been killed in fierce fighting with Muslim militants. ( AP: Aaron Favila )

Strained relationship for allies

A US P3 Orion surveillance plane was seen flying over the town on Friday (local time), but there has been no evidence that the United States has put troops on the ground there.

The assistance comes after months of strain between the two long-time allies that was stoked by Philippines President Rodrigo Duterte's hostility towards Washington and his pledges to throw US troops out of the country.

Washington deployed special forces soldiers to Mindanao in 2002 to train and advise Philippine units fighting ASG militants in a programme that once involved 1,200 Americans.

It was discontinued in 2015 but a small presence remained for logistics and technical support.

Thousands of Marawi residents fled after Muslim militants sieged the city in southern Philippines. ( AP: Bullit Marquez )

The United States and the Philippines have been allies for decades.

Their relationship provided Washington with a strategic foothold in Asia, and offered Manila a shield against China's assertiveness in the region.

But Mr Duterte has openly scorned the alliance, seeing it as an obstacle to a rapprochement with China, and has repeatedly lambasted Washington for treating his country as a lackey.

Reuters