"They are not fighting".
US special forces are providing support to the military battling to reclaim the southern city, which has been partly under Islamist militant control for roughly two and a half weeks.
Twelve other Marines were killed during Friday's skirmishes, Herrera said.
"Initial reports indicate that the fierce firefight ensued when the Marines assaulted the reported enemy position at around 3:30 a.m. and firefight continued up to around 5 p.m. of the same day", he said in a statement.
The jihadists overran the city on May 23: in the ensuing battle, 58 troops, 138 militants and 20 civilians died.
Philippine military spokesman Lieutenant Colonel Jo-ar Herrera confirmed the U.S. help, adding that the special forces were not fighting, but "providing technical support".
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.
The Two-Way has reported that an alliance of extremist groups have held out against the siege using supplies stored in civilian sites such as mosques and schools, as well as a network of underground tunnels.
On Friday, police also arrested the mother of the Maute brothers, the two men leading the fighters in Marawi City.
"We are making adjustments in our offensives", he said, adding that air strikes were effective in the operations.
The Armed Forces of the Philippines (AFP) denied on Saturday the presence of American troops fighting alongside Filipino government forces here.
The military hopes to "liberate" the besieged city by June 12 from the militants.
US Special Operations Forces "have been providing support and assistance in response to Philippine government requests for many years", the embassy said.
"We don't have adequate surveillance equipment, so we asked the U.S. military for assistance".
"At the same time, we commend the efforts of the peace panels of the government and the Moro Islamic Liberation Front for brokering a humanitarian ceasefire that allowed for the safe passage and rescue of some 130 civilians, as well as the pool of volunteers who were part of this charitable initiative", it said.