Thirteen Philippine marines were killed and at least 40 others were wounded in a 14-hour clash with militants linked to the Islamic State in the city of Marawi, the military said on Saturday.
It was believed to be the deadliest single-day toll for military forces battling Islamist militants from the Abu Sayyaf and Maute groups that seized part of the city in May, beginning a wave of violence across Marawi and displacing tens of thousands of Filipinos.
The battle erupted at dawn on Friday after the marines targeted militants holding several civilian hostages, and the fighting stretched into dusk. The clashes were still continuing on Saturday, said Col. Edgard Arevalo, a military spokesman.
“The marines were able to inflict heavy casualties to the terrorist group,” Colonel Arevalo said.
The militants’ siege has become the greatest test of the year-old presidency of Rodrigo Duterte, who previously dismissed the Maute group as bandits who could be easily defeated. The fighting led Mr. Duterte to impose martial law over the southern Philippines in May.
While the Philippine president had vowed to end an alliance with the United States, his government has sought the aid of United States special forces to try to end the Marawi siege.
In a statement on Saturday, the United States Embassy said, “At the request of the government of the Philippines, U.S. special operations forces are assisting the (Filipino military) with ongoing operations in Marawi.”
The embassy declined to provide further details, citing the sensitivity of the operation, but it said Washington remained committed to addressing “shared threats,” including counterterrorism issues.
Among government forces killed on Friday near Madaya, one of the villages in Marawi seized by the militants, was Lt. Frederick Savellano, who in the past week had led a team that recovered about 52.2 million pesos (more than $1 million) believed to have been abandoned by the militants, Colonel Arevalo said.
“This temporary setback has not diminished our resolve a bit,” he said, adding that it had made the military more determined “to neutralize the enemy, save the innocent lives trapped in the fight and set the conditions for the rehabilitation and reconstruction of Marawi.”
Philippine troops have been battling to clear Marawi since May 23, when the militants — led by Isnilon Hapilon, the head of Abu Sayyaf, and backed by members of the Maute group and the foreign fighters supporting them, laid siege to the area. But the insurgents have broken up into small groups and have proved elusive.
Mr. Hapilon is the de facto leader of the Islamic State in the Philippines, while the Maute group, led by two brothers, has been linked to a series of bombings in the restive south, including one targeting a night market in Mr. Duterte’s southern hometown that killed 15 people in September.
On Saturday, Colonel Herrera said there was a “strong indication” that the Maute brothers, Abdullah and Omarkhayam, were killed in the fighting on Friday.
“We are still validating those reports,” he said. No further information was available.
The killings on Friday led Mr. Duterte to cancel an annual gathering of foreign diplomats scheduled for Monday, the country’s 119th Independence Day, to meet with his generals.
The death toll for government forces since the fighting began now stands at 58. Twenty-four civilians and at least 120 militants have been killed in the fighting, which has transformed large parts of the once-vibrant Islamic city of Marawi into rubble.
Tens of thousands of people have also been displaced and need emergency assistance, rescuers say.
Lt. Col. Joar Herrera, a military spokesman in Marawi, said on Saturday that the marines had been conducting house-to-house searches when they encountered the Maute group holding hostages. Snipers shot at the marines, and a raging battle ensued.
Colonel Herrera said that about 40 militants had positioned themselves in the mosques and had placed bombs in strategic places.
“They were using the mosques as their safe haven” and using the hostages as “human shields,” he said of the terrorist group.
Also on Friday, the Maute brothers’ mother, Ominta Romato Maute, also known as Farhana, was arrested with nine others, including two wounded militants, in the nearby town of Masiu outside Marawi.
She had been buying vehicles and firearms for an escape from the province, reports said. The authorities seized several high-powered firearms and an improvised bomb from the group, the police said.
Her husband, Cayamora Maute, and several others were arrested on Tuesday trying to enter the southern city of Davao, Mr. Duterte’s hometown. He was taken to detention in Manila.
