The U.S. Embassy in Manila said without elaborating that U.S. special operations forces were providing help to Filipino troops battling the Maute and Abu Sayyaf militants in Marawi.
In a press briefing, Lt. Col. Jo-ar Herrera said that USA special forces were helping the army.
The Filipino military has called on Facebook to close 63 accounts linked to ISIS-affiliated Maute militants engaged in the fighting.
He said they were killed during an "intense firefight" on Friday as they were conducting operations to clear enemy positions.
The heavy fighting occurred as police arrested the mother of two brothers leading the militants, who come from different local groups that have pledged allegiance to the Islamic State terrorist movement.
Tens of thousands have fled Marawi, which is the Catholic country's most important Muslim city, as the fighting ripped through residential neighbourhoods, with more than 200,000 displaced people now living in precarious conditions and vulnerable to deadly health risks, the International Committee of the Red Cross warned Friday.
Herrera said the number of troopers killed is "part and parcel of our offensive operations".
They initially estimated there were about 100 gunmen but later said there were as many as 500, supplemented by foreign fighters from Chechnya, Saudi Arabia and Yemen.
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An Associated Press journalist and photographer saw a U.S. Navy P3 Orion plane hovering in cloudy skies above Marawi on Friday.
Until now there had been no confirmation that USA forces were assisting in the battle for Marawi City, which is in its third week. "They are just providing technical support", he said.
President Rodrigo Duterte had earlier threatened to throw out United States troops amid strained relations since taking office.
FILE PHOTO: Smoke billowing from a burning building is seen as government troops continue their assault on insurgents from the Maute group, who have taken over large parts of Marawi City.
It was discontinued in 2015 but a small presence remained for logistics and technical support.
Without confirming the specific request from the Philippines, a Facebook spokesman said the company has "well-established law enforcement channels for governments to contact us about emergencies".
But 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.