A US-led coalition air strike killed 20 civilians, including two children, overnight in a village near the ISIS group's Syrian stronghold of Raqqa, a monitor said Wednesday.
The coalition confirmed that it had carried out strikes in the area, but said it was investigating whether they had killed civilians.
The Britain-based Syrian Observatory for Human Rights said dozens of people were also wounded in the strikes Tuesday night on the village of Al-Heisha, 40 kilometers (25 miles) north of Raqqa.
"The toll has risen to 20 civilians, including nine women and two children," Observatory chief Rami Abdul Rahman said.
The monitor had earlier given a death toll of 16.
Abdul Rahman said 32 people were also injured in the strikes, adding that all the casualties were civilians.
The Observatory said the latest deaths brought the number of civilians killed since US-led air strikes in Syria began in September 2014 to 680, including 169 children.
Al-Heisha is controlled by ISIS and has been a target in a new assault by the US-backed Syrian Democratic Forces (SDF) as it pushes to capture the militants' bastion of Raqqa.
Colonel John Dorrian, a spokesman for the coalition, told AFP that an "initial assessment" confirmed there had been strikes in the area.
"After an initial assessment of strike logs against the date and location of the alleged civilian casualties, the coalition confirms it did conduct strikes in the area described in the allegation," he said.
"However, more specific information is needed to conclusively determine responsibility."
He said investigations would continue into the incident and stressed that coalition forces adhere to "stringent targeting processes and procedures" to minimize civilian casualties.
A spokeswoman for the Kurdish-Arab alliance, however, dismissed the reports of civilian deaths.
"There is no such thing, and any such claims are IS news," Jihan Sheikh Ahmed told AFP.
The SDF's media account said at least six ISIS fighters were killed by coalition air strikes on the village and accused the extremist group of preventing civilians from leaving Al-Heisha in a bid to use them as "human shields".
The SDF announced Sunday it had begun an operation to capture Raqqa from ISIS, as the jihadist group also comes under pressure across the border in Iraq, where its bastion Mosul is under attack.
Civilians fleeing Al-Heisha told AFP on Tuesday that ISIS fighters had brought heavy weaponry into the village when the SDF operation began.
"Daesh (ISIS) fighters brought heavy weapons to our village and stayed among us so that if there were strikes they would hit us," 45-year-old Saada al-Aboud said.
"They wouldn't let us leave. We had to escape by running out into the fields, with our children and old people. What else could we do? We left everything behind."SHOW MORE