A car bomb went off Saturday near a prison holding ISIS extremists in northeastern Syria, where Turkey is pursuing an offensive, a war monitor and a Kurdish official reported, according to the German news agency (dpa). The bombing took place outside the central prison in the district of Ghuwaran, in the northeastern city of al-Hasakeh, the Britain-based Syrian Observatory for Human Rights added. The Kurdish-led Syrian Democratic Forces (SDF) rushed military reinforcements to the prison to prevent ISIS detainees from escaping, the watchdog added. No casualties were reported. SDF spokesman Mustafa Bali blamed ISIS for the attack. The reported bombing comes a day after at least three civilians were killed in a car bombing claimed by ISIS in the city of Qamishli, in northeastern Syria. On Wednesday, Turkey started an incursion into northeastern Syria, saying it is targeting ISIS extremists and Kurdish militias, whom Ankara considers to be linked to the outlawed Kurdistan Workers Party (PKK) waging an insurgency within the country. Syrian Kurdish authorities are already struggling to guard ISIS fighters captured during the long US-backed campaign against the extremists and to keep a lid on ISIS supporters and family members thronging displacement camps. Their hold will suffer even more as they fight Turkey. The White House has said Turkey will take over responsibility for the thousands of imprisoned fighters. But it is not clear how that will happen. Kurdish authorities run more than two dozen detention facilities, scattered around northeastern Syria, holding about 10,000 ISIS fighters. Among the detainees are some 2,000 foreigners, including about 800 Europeans. Most of the facilities are unidentified and unmarked, some of them set up in abandoned or re-purposed buildings; others are mobile "pop-up prisons." Some are reportedly close to the border, which may make them vulnerable to being hit in clashes or bombardment. Guarding those facilities has long been a strain on the SDF as it juggles multiple tasks in the volatile area. Despite its territorial defeat, ISIS has maintained an insurgency in Iraq and Syria, carrying out suicide bombings, assassinations and ambushes. Some reports suggest 14,000 to 18,000 ISIS members remain in Syria and Iraq, including 3,000 foreigners, but a recent report by the inspector general of the Department of Defense said figures from experts greatly vary. ISIS militants have carried out 80 to 90 attacks in Kurdish-held areas of Syria. In August alone, ISIS militants claimed 78 attacks, according to the Rojava Information Center, including the assassination of Kurdish fighters and explosive devices planted near patrols. The group has also been reviving financial networks through extortion, "taxing" the local population or collecting ransoms from kidnapping.