The extremists occupied a hospital, jail and other buildings in Marawi City last month.
A 15-year-old boy was also killed by a stray bullet as he took part in Friday prayers at a mosque in Marawi as the skirmishes raged outside, an AFP video journalist said.
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.
The Armed Forces of the Philippines have been slugging it out with Maute militants for control of Marawi, in the southern Mindanao region of the Philippines.
Days before the United States embassy issued its statement, Herrera had already confirmed that the United States was providing "techical assistance" to Philippine troops battling local terrorists in Marawi City.
Herrera said security forces were struggling to contain the fighters without causing civilian casualties.
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. A US P3 Orion surveillance plane was seen flying over the town on Friday, but there has been no evidence that the United States has put troops on the ground there.
The cooperation between the longtime allies is significant because President Rodrigo Duterte, who came to power a year ago, has taken a hostile stance toward Washington and has vowed to eject US military trainers and advisers from his country.
Washington deployed special forces soldiers to Mindanao in 2002 to train and advise Filipino units fighting Abu Sayyaf militants in a program that once involved 1200 Americans.
It was discontinued in 2015 but a small presence remained for logistics and technical support.
The insurgents have so far withstood more than two weeks of air and ground assaults by security forces, with about 2,000 people believed to be trapped in militant-held areas although the military said this amounts to only around 10% of the city.
The latest death toll in the ongoing fight for Marawi is 217, including 58 from the government side, 138 militants, and 21 civilians.