JAKARTA (Reuters) - Indonesian police on Friday (Aug 11) arrested an alleged recruiter and fundraiser for pro-Islamic State in Iraq and Syria (ISIS) militants locked in a bloody battle for control of the southern Philippine city of Marawi.

Nearly 700 people, 120 soldiers among them, have been killed in the conflict after an alliance of militant groups launched an audacious assault to capture the Philippine city on May 23.

Philippine military and police have yet to regain control of all of Marawi, a city of about 200,000 people, amid ferocious urban fighting that has destroyed most of its centre.

The man detained at a residential complex on the outskirts of Jakarta is believed to be a member of Jemaah Ansharut Daulah (JAD), an Indonesian radical group that has pledged allegiance to ISIS, police spokesman Inspector General Setyo Wasisto said.

"He finds people to send to Marawi and Syria," Wasisto said in a text message to Reuters. "How many is still unclear."

Police also suspect he raised funds for recruitment.

Indonesian counter-terrorism authorities believe at least 20 Indonesians were among the fighters, along with some from Malaysia and the Middle East, who flocked to Marawi, on the island of Mindanao, long afflicted by Islamist insurgencies.

Indonesian JAD members make up most of the senior leadership of the South-east Asian military unit fighting for ISIS in Syria known as Katibah Nusantara.

Two of the leaders, Bahrumsyah and Bahrun Naim, have directed and inspired a series of militant attacks in Indonesia, Indonesian police say.

Members of Katibah Nusantara have also organised funding and international recruits for the Marawi assault, the Jakarta-based Institute of Policy Analysis of Conflict said in a report.

South-east Asian nations have vowed to step up law enforcement and intelligence cooperation to fight the rising threat of violent Islamist extremism in the wake of the Marawi siege.

Fuelling concern is the possible return to Southeast Asia of hundreds of hardened ISIS fighters from the Middle East as its self-styled caliphate collapses.