MANILA--Philippine security forces on Sunday arrested the Indonesian wife of the slain pro-Islamic State militant leader who planned and led the attack on Marawi city, authorities said.

President Rodrigo Duterte declared last month the liberation of Marawi from the Maute group, a pro-Islamic State militant group which held the lakeside town after the death of two top leaders, including the woman's husband Omarkhayam.

The army announced the termination of combat operations a week later, ending a five-month battle that killed more than 1,100 people, including 165 soldiers. Close to 400,000 people were also displaced as fighting destroyed the city.

National Police Chief Ronald dela Rosa said bomb-making materials were seized from Minhati Madrais, who was with her six children when security forces raided her home in Iligan city.

"Her children, who are all minors, will be turned over to the social welfare department," he said, adding that rebellion and illegal possession of explosive devices charges were now being prepared against her.

He said Madrais, alias Baby, was among more than 100 people ordered arrested by Defense Secretary Delfin Lorenzana after martial law was declared by Duterte on May 23. Madrais' passport is also expired, another violation of Philippine laws, he added.

Sporadic gunfire were still heard in Marawi on Sunday, almost two weeks after combat operations were formally terminated, Col. Romeo Brawner, deputy commander of Task Force Ranao, told reporters.

"We are still hunting down stragglers," he said, adding there could be as many as 39 militants hiding in the ruined city based on information from captured Indonesian militants.

"We do not have information how many militants are still there. But, we will hunt them down."

The new army commander, Lt.-Gen. Rey Leonardo Guerrero, said he would work closely with Indonesia and Malaysia to step up coordinated sea and air patrols along the borders where kidnappings and attacks on shipping are happening.

"We intend to increase the frequency of our coordinated patrols with our ASEAN neighbors to stop piracy and militant attacks in these dangerous seas," he said.