Philippine troops are struggling to dislodge hundreds of fighters, who rampaged through the mainly Muslim city of Marawi on May 23 flying black flags of the Islamic State group, and have used bomb-proof tunnels, anti-tank weapons and human shields to fortify their positions.
Lieutenant Colonel Jo-ar Herrera said the military was verifying the number of troops wounded in Friday's ferocious, sometimes house-to-house gunbattles with the militants.
"We are saddened with the result... we have fatalities on the government side. We have incurred 13 killed in action," Herrera said at a news conference in Marawi.
Australia 'ready to assist' the Philippines in IS battle
The fresh casualties brought to 58 the number of government troops killed in the fighting, Herrera said.
At least 138 militants and 20 civilians have also been killed, the government said.
The militants have so far withstood more than two weeks air and ground assaults by security forces, although the military said they occupy only around 10 per cent of the city.
US forces assist Philippines in battle to end city siege
US forces are providing the Philippines with technical assistance to end the siege in Marawi but it has no boots on the ground, the Philippines military said on Saturday.
The seizure of Marawi by hundreds of fighters who have sworn allegiance to IS, including dozens from neighbouring countries and the Middle East, has fuelled concern that the ultra-radical group is gaining a foothold in Southeast Asia.
Earlier a US embassy spokesperson in Manila told Reuters that, at the request of the Philippines government, special operations forces were helping liberate the town, part of which
has been occupied by hundreds of militants since May 23.
In Marawi, military spokesman Lieutenant Colonel Jo-Ar Herrera confirmed the U.S. assistance, telling a news conference: "They are not fighting. They are just providing technical support."
A US P3 Orion surveillance plane was seen flying over the town on Friday, according to local media reports.
Until now there had been no confirmation that the Philippines had sought US support in the battle for Marawi City on the island of Mindanao, which is in its third week.
The assistance comes after months of strain between the two long-time allies that was stoked by Philippines President Rodrigo Duterte's hostility towards Washington and his pledges to throw US troops out of the country.
Washington deployed special forces soldiers to Mindanao in 2002 to train and advise Philippine 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 United States and the Philippines have been allies for decades. Their relationship provided Washington with a strategic foothold in Asia, and offered Manila a shield against China's assertiveness in the region.
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.