Five people, including a Kurdish fighter, were killed on Monday in an ISIS suicide bombing in Syria’s Raqqa city, said the Syrian Observatory for Human Rights. The attacker blew himself up after entering the center run by the Kurdish Peoples Protection Units (YPG), said the Britain-based monitor. "A suicide attacker wearing an explosives belt blew himself up inside a YPG center after opening fire on a security checkpoint at its entrance," Observatory chief Rami Abdel Rahman said. At least four civilians and a YPG fighter were killed in the attack, he added. The YPG forms the backbone an alliance of Kurdish and Arab fighters called the Syrian Democratic Forces (SDF) battling to expel ISIS from its last redoubt in eastern Syria. ISIS claimed responsibility for the attack they said targeted a "recruitment center" for Kurdish forces, reported AFP. "The attacker aimed at them with a machine gun then blew up his explosives vest in the middle of them," it said in a statement on the Telegram messaging app. Raqqa, which ISIS declared the capital of its self-proclaimed caliphate, was taken by the SDF six years into Syria’s war after a protracted battle that killed thousands of civilians and wrecked much of the city. Rebuilding and restoring Raqqa, which was once home to more than 200,000 people, is proving to be a monumental task. Since losing almost all the large swathes of territory it took in lightning offensives in Syria and Iraq in 2014-15, ISIS has returned to deadly hit-and-run attacks and ambushes.