Thirteen Philippine marines were killed in fierce fighting with Muslim militants who have laid siege to a southern city for nearly three weeks in the biggest single-day loss for government forces, the military said on Saturday.

A U.S. Navy aircraft provided surveillance for the local troops as the battle raged in Marawi on Friday, confirming the involvement of the U.S. military in helping quell the urban insurrection at the request of the Philippine government, Philippine military officials said.

An Associated Press journalist and photographer saw a U.S. Navy P3 Orion plane hovering in cloudy skies above Marawi on Friday. The aircraft flew above rocket-firing Philippine helicopters that struck militant positions, causing plumes of smoke to billow skyward.

“We don’t have adequate surveillance equipment, so we asked the U.S. military for assistance. It’s non-combat assistance,” military spokesman Brig. Gen. Restituto Padilla said by phone, citing a Philippine government policy that bars foreign troops from local combat.

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.

Philippine marines were conducting a house-to-house search for militants allied with the Islamic State group who are still occupying parts of Marawi when the battle erupted Friday, said Lt. Col. Jo-ar Herrera, spokesman for the Philippine army’s 1st Infantry Division.

About 30-40 militants used civilians as human shields, making it hard for troops to operate, and also positioned themselves in the city’s many mosques, he said.