Meanwhile, Herrera confirmed the participation of U.S. military special forces in the government's fight against the Maute.
Both Maute and Abu Sayyaf have declared their allegiance to ISIL.
A Philippine military spokesman also said there were "strong indications" that the Maute brothers Omar and Abdullah, who founded the militant group named after them, were dead.
It was among the heaviest fighting in the standoff, according to AFP journalists in Marawi, with the air force supporting ground troops with sustained bombing runs that battered the city.
"There was an intense, house-to-house fire fight between the marines and the local terrorist group", Jo-Ar Herrera, army spokesman, told a televised press conference.
Up to 1,000 civilians are believed to be trapped in an area of the city controlled by at least 200 militants.
Herrera said the number of troopers killed is "part and parcel of our offensive operations".
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. The two were detained on suspicion of providing financial and other support to their children who are involved in the fighting in Marawi, officials said.
In Marawi, military spokesman Lieutenant Colonel Jo-Ar Herrera confirmed the USA assistance, telling a news conference: "They are not fighting".
The US troops are not fighting, but an American surveillance plane was seen flying over the city on Friday.
Police said one of the latest fatalities was a 15-year-old boy killed by a stray bullet attributed to a militant sniper while praying inside a mosque.
The assistance comes after months of strain between the two long-time allies that was stoked by Philippine President Rodrigo Duterte's hostility towards Washington and his pledges to throw U.S. troops out of the country.
It is the biggest loss for government forces in a single day, the military said.
"The presence of our USA counterparts facilitates exchanges of intelligence ... and also provides training exchanges and, of course, the technical support that we are sharing together in the fight against global terrorism", he added.
The US Embassy said Washington would "continue to work with the Philippines to address shared threats to the peace and security of our countries, including on counterterrorism issues" in its statement Saturday.