A US-backed Kurdish-Arab alliance has pushed closer to the Syrian city of Raqqa and forces in Iraq have seized a key town near Mosul as offensives against Islamic State militants continue.

After announcing the start of the long-awaited operation to retake Raqqa on Sunday, the Syrian Democratic Forces alliance said it had moved south towards the city despite fierce resistance from militants.

Meanwhile south of Mosul, Iraqi forces have retaken the town of Hamam al-Alil, a key objective in their three-week advance on the city. Mosul is the last major ISIL stronghold in Iraq while Raqqa is one of the few key bases left under militant control in Syria.