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

A US Navy aircraft provided surveillance for the local troops as the battle raged in Marawi on Friday, confirming the involvement of the US 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 US 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 US military for assistance. It's noncombat assistance,” military spokesman Brig Gen Restituto Padilla said by phone, citing a Philippine government policy that bars foreign troops from local combat.

The US Embassy in Manila said without elaborating that US special operations forces were providing help to Filipino troops battling the Maute and Abu Sayyaf militants in Marawi.

“The United States is a proud ally of the Philippines, and we will continue to work with the Philippines to address shared threats to the peace and security of our countries, including on counter-terrorism issues,” the embassy said in a statement.

Philippine marines were conducting a house-to-house search for militants allied with the militant 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.

Forty other marines were wounded, Herrera said.

Philippine military officials say the violence has left at least 138 militants and 58 government troops dead. At least 21 civilians have been killed, including a boy who was hit by suspected militant gunfire inside a Marawi mosque where his family had taken refuge, Padilla said.

Hundreds of thousands of people have fled the city, parts of which were reduced to rubble by fighting and government airstrikes in an attempt to dislodge the rebels.

“This temporary setback has not diminished our resolve a bit,” said military spokesman Col Edgard Arevalo.

“It instead primed up our determination to continue our prudent advances to neutralise the enemy, save the innocent lives trapped in the fight, and set the conditions for the rehabilitation and reconstruction of Marawi.” Arevalo vowed.

Filipino forces, meanwhile, captured on Friday the mother of two top militant leaders leading the siege.

Ominta Romato Maute, who is also known as Farhana, was arrested with two wounded men and several woman allegedly with assault rifles and other weapons in Masiu town in Lanao del Sur province.

Maute's husband, Cayamora, was arrested at a police checkpoint in the southern city of Davao on Tuesday.

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.

Philippine President Rodrigo Duterte has declared martial law in the Mindanao region, the southern third of the Philippines and home to a decades-long Muslim separatist rebellion.

Defense Secretary Delfin Lorenzana, the designated administrator of martial law, has ordered the arrest of nearly 200 militants, politicians and other suspected civilian backers of the unprecedented uprising in Marawi, the mosque-studded heartland of Islamic faith in the south of the predominantly Roman Catholic nation.

The Marawi siege followed a May 23 army raid that failed to capture a top terror suspect, Isnilon Hapilon, who has been designated by the Islamic State group as its leader in Southeast Asia.

The raid, however preempted a plot by hundreds of militants waving Islamic State group-style black flags to capture Marawi and kill Christians, military officials say.