The US-led coalition against the Islamic State group says it has carried out a series of air strikes in Syria on its main stronghold, Raqqa.

The city is seen by the militants as the capital of the "caliphate" they declared in Syria and Iraq in 2014.

The US military described the 16 strikes as one of the largest assaults carried out in Syria so far.

The Syrian Observatory for Human Rights said at least 23 IS members were killed in the attack.

The London-based monitoring group also said a US drone strike on a Raqqa school on Saturday killed six civilians, including a child.

US military spokesman Lt Col Thomas Gilleran said: "The significant air strikes tonight were executed to deny Daesh [IS] the ability to move military capabilities throughout Syria and into Iraq."

BBC