At least 56 civilians have died in US-led coalition air strikes near the Islamic State stronghold of Manbij in north Syria, opposition monitors say.
The Syrian Observatory for Human Rights said residents had been fleeing the village of Tokhar when they were hit.
An opposition activist network said 90 had died in Tokhar and nearby Hoshriya.
There was no immediate comment from the coalition, which has been providing air support for the Kurdish-led offensive to drive IS militants out of Manbij.
On Monday, the Syrian Observatory and another monitoring group, the Local Co-ordination Committees (LCC), both said coalition air strikes in the Tokhar area had caused 15 deaths.
The Syrian Observatory said 11 children were among those killed as people fled the village, which is about 14km (9 miles) north of Manbij, at dawn on Tuesday.
The UK-based monitoring group's director, Rami Abdul Rahman, told AFP news agency that the strikes appeared to have been carried out in error, with the civilians mistaken for IS militants.
The LCC said "vacuum missiles" had been fired at Tokhar and Hoshriya, an apparent reference to fuel-air explosive munitions.
The opposition activist network put the death toll in the two areas at 90 and said it included whole families.
An alliance of Kurdish and Arab fighters called the Syria Democratic Forces (SDF) began an operation to capture Manbij and cut critical supply routes to the IS-held city of Raqqa at the end of May.
Under the cover of US-led coalition air strikes, they succeeded in encircling Manbij within days and entered the town's outskirts in late June.
However, the SDF's advance has been slowed in recent weeks by landmines planted by IS and suicide bomb attacks.
On Friday, UN High Commissioner for Human Rights Zeid Raad Al Hussein warned that the situation in Manbij, where an estimated 70,000 civilians were trapped, was believed to be deteriorating dramatically.
Mr Zeid said his office had received reports of a number of civilians, including women and children, being killed and injured by air and ground strikes, as well as by landmines.
"Civilians have also reportedly been killed if they leave their homes or attempt to flee," he added.
"Families are unable to access local cemeteries to bury their relatives... and are burying them in their gardens or keeping the corpses in bunkers."
"The town has no electricity or water at present, and no medical facilities are known to be operating," the UN official added. "As the SDF closes in on the city, [IS] has not permitted civilians to leave the area."