At least 56 civilians have been killed in air strikes north of the besieged Islamic State-held city of Manbij in northern Syria, and residents say they believe the attack was carried out by US-led warplanes, a monitoring group says.
The Britain-based Syrian Observatory for Human Rights said Tuesday's dead included 11 children, and that dozens more people were wounded.
The US-backed Syria Democratic Forces (SDF), an alliance of Kurdish and Arab fighters, launched an offensive at the end of May to seize the last territory held by Islamic State (IS) insurgents on Syria's frontier with Turkey.
Supported by US coalition air strikes, the SDF have surrounded the city, but Islamic State attacks still occur in some areas of the surrounding countryside.
On Monday, 21 people were killed in raids also believed to have been conducted by US-led coalition aircraft on Manbij's northern Hazawneh quarter.
But progress into Manbij city has been slow. The militants have deployed snipers, planted mines and prevented civilians from leaving, hampering efforts to bomb the city without causing heavy casualties, according to Kurdish sources.
The Observatory said at least 104 civilians have died from air strikes since the start of the Manbij offensive in late May.
Colonel Chris Garver, a spokesman for the US coalition against Islamic State in Syria and Iraq, said it was looking into reports of civilian deaths but was being "extraordinarily careful to make sure" air strikes were killing IS fighters.
"Around Manbij, the Syrian Arab Coalition (SAC - Arab groups within the SDF), which is leading that fight, is being very slow and deliberate in that fight to protect civilians which we know are inside."
The UN High Commissioner for Human Rights recently voiced concern for the roughly 70,000 civilians believed to be trapped between warring parties in Manbij.
"Civilians have... reportedly been killed if they leave their homes or attempt to flee. Families are unable to access local cemeteries to bury their relatives who have died or been killed, and are burying them in their gardens or keeping the corpses in bunkers," Zeid Ra'ad Al Hussein said.
"The town has no electricity or water at present, and no medical facilities are known to be operating. As the SDF closes in on the city, [Islamic State] has not permitted civilians to leave the area."