The attack happened in al-Ghandour near Manbij, just one day after the U.S. announced a formal inquiry into airstrikes in the same area that may have led to the single largest loss of civilian life due to coalition airstrikes in Syria.
The death toll from Thursday's attack is expected to rise because some civilians are in critical condition, according to the observatory, which says it knows of an additional 13 killed who have yet to be identified.
"We can confirm the Coalition conducted airstrikes in the area in the last 24 hours. As with every report of civilian casualties, we will review any information we have about the incident, including information provided by third parties, such as the proximity of the location to Coalition airstrikes, and any other relevant information presented. If the information supporting the report is determined to be credible, we will then determine the next appropriate step," the statement released Thursday said.
The U.S. Central Command also announced Thursday that from July 28, 2015, to April 29, 2016, six separate U.S. airstrikes in Syria and Iraq resulted in the death of 14 civilians.
"The U.S. rarely concedes any of these mass casualty events, despite often very compelling public evidence," Chris Woods, director of Airwars, told ABC News.
On Wednesday, Colonel Christopher Garver, spokesperson for the coalition against ISIS, named Operation Inherent Resolve, told members of the press that the U.S. has initiated a formal investigation into the July 19 airstrikes after finding the information available about the attack credible enough to warrant such an inquiry.
Garver also said that the U.S. is assessing reports that civilians were killed in other U.S. airstrikes on the village of al-Nawaja, east of Manbij, on July 23. In those strikes, 22 civilians were reported killed, according to Airwars.
Since August 2014, between 773 and 1,180 civilians have reportedly been killed by coalition airstrikes in Syria, according to Airwars.