At least 85 civilians, including almost a dozen children, have been killed after US air strikes targeted an area in northern Syria held by Islamic State (Isis/Daesh) fighters, a monitoring group has said.

Some eight families were reportedly wiped out in the IS-controlled village of Tokhar near Manbij on Tuesday (19 July), in what could be one of the deadliest bombings of civilians by coalition forces since the start of operations in the country.

The Turkey-based Syrian Institute for Justice said it feared the death toll could be far higher as rescuers continued to pull bodies from the rubble. Photographs of the aftermath showed several half-buried children being retrieved.

The US military confirmed it had conducted air strikes in the area and said it was investigating claims of civilian deaths.

"If the information supporting the allegation is determined to be credible, we will then determine the next appropriate step," the statement said. It added that the US military takes "all measures during the targeting process to avoid or minimise civilian casualties".

A second monitoring group, the UK-based Syrian Observatory for Human Rights, believed the air strikes were carried out in error after pilots mistook the civilians for IS fighters. It also reported a separate incident, on Sunday, in which six civilians in Manbij were killed by a coalition air strike, "including a woman with four of her children and an old man".

Manbij and surrounding villages have seen intense fighting in recent weeks with US air strikes helping Kurdish-led ground forces attempt to retake the besieged city from the terrorist group, which it has held since 2014.

Earlier on Tuesday, US military officials announced the US-backed Syria Democratic Forces (SDF), a mix of Kurdish and Arab fighters, had taken control of an IS headquarters in the area, located in a hospital.

It revealed the US had carried out more than 450 air strikes around Manbij since its coalition offensive began at the end of May.

Efforts to enter the city centre have been painfully slow, however, with IS militants staging counterattacks and using mines and snipers to pin forces back. Thousands of civilians have fled the city but many more remain, with IS militants accused of using them as human shields.

The capture of Manbij, part of a wider operation to secure the Syria-Turkey border, would deliver a serious blow to IS forces. The terrorist group uses the city in a supply line between the Turkish border and the capital of the self-declared Islamic State, Raqqa.