U.S. special forces have joined an operation to oust Islamic State-affiliated militants who have laid siege to a city in the southern Philippines.

"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.

Friday's ferocious, street-to-street gunbattles with the militants saw 13 troops killed, in a dramatic surge in the toll from the conflict, Philippine military spokesmen said.

In more than two weeks of fighting, at least 202 people have been killed: 134 militants, 38 troops, and 30 civilians. In 2002, the United States helped Philippine authorities track down Abu Sayyaf leader Abu Sabaya, who was killed in that operation.

Philippine Army Scout Rangers take positions during a mission to flush out Islamist militant snipers in Marawi on June 6, 2017.

Gen. Restituto Padilla said by phone, citing a Philippine government policy that bars foreign troops from local combat.

Herrera said Farhana has been transported away from Marawi City to a "safe place", and is now currently undergoing investigation.

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Military casualties stemmed from intense, house-to-house fighting and the militant's use of improvised explosive devices, Herrera added.

A U.S. Navy aircraft provided surveillance for the local troops as the battle raged in Marawi on Friday, confirming the involvement of the U.S. military in helping quell the urban insurrection at the request of the Philippine government, Philippine military officials said.

Debris fly as Philippine Air Force fighter jets bomb suspected locations of Muslim militants as fighting continues in Marawi city, southern Philippines, Friday, June 9, 2017.

In Marawi, military spokesman Lieutenant Colonel Jo-Ar Herrera confirmed the USA assistance, telling a news conference: "They are not fighting".

But Herrera denied that the United States troops are not joining Filipino soldiers in the fighting on the ground.

The BBC's Jonathan Head says there are several reports that the Maute brothers, who lead the Maute group, are among the dead, with intercepted communications from jihadist groups suggesting this.

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