French Defense Minister Florence Parly said Friday that the battle to flush ISIS militants of their last holdouts in the northern Syrian city of Raqqa could drag on for "weeks.” "Its probably a question of weeks," Parly told France Inter radio. "Its a slow, difficult battle but which is nonetheless effective," she said, adding the continuing fight for the city center was "obviously the hardest.” The US-backed Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, stormed ISIS’ Syrian bastion in June and has since wrested 90 percent of the city from the terrorist group. France is part of the US-led coalition that has been pounding ISIS positions in Iraq and Syria from the air in order to help local forces flush out the extremists. Parly estimated that around 500 French militants remained in Iraq or Syria. "Many are thrown onto the frontlines,” she said, adding those who want to flee are forced by ISIS to fight. Aid agencies said Thursday that Syria is in the throes of its worst fighting since the battle for eastern Aleppo last year, with heavy air strikes causing hundreds of civilian casualties. Hospitals, schools and people fleeing violence have been "targeted by direct air strikes" that may amount to war crimes, the United Nations said, without apportioning blame. Russia and the US-led coalition are carrying out separate air strikes in Syria ostensibly aimed at defeating ISIS. The International Committee of the Red Cross (ICRC) also said in a statement that up to 10 hospitals were reported to have been damaged in the past 10 days.