The main Syrian opposition group had urged the US-led coalition to suspend its bombardments following the July 19 strikes, which a rights-monitoring group and local residents said had killed dozens of civilians.
After examining "internal and external information," the coalition determined that there was sufficient credible evidence of civilian victims to open a formal inquiry, said spokesman Colonel Chris Garver.
The Britain-based Syrian Observatory for Human Rights said at least 56 civilians, including 11 children, died as they fled from a village near Manbij, a strategic waypoint between Turkey and the jihadist stronghold of Raqa.
Garver said yesterday that death estimates from residents near Manbij ranged from a low of "10 to 15" to a high of 73.
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Garver had earlier accused the Islamic State of using civilians as "human shields."
Coalition officials often say theirs is the most precise air campaign in history.
Nearly all coalition air strikes use guided munitions, involving laser or GPS systems, or else missiles. Targets are often viewed at length using surveillance drones before the order to attack is issued.
The London-based non-governmental organisation Airwars has estimated that the roughly 14,000 coalition bombing attacks since August 2014 have claimed at least 1,513 civilian lives.
The coalition has officially acknowledged only a few dozen civilian victims.
After the air strikes of July 19, the main Syrian opposition group, the Istanbul-based National Coalition, called on the US-led forces to suspend bombardments.
The group's president, Anas al-Abdeh, said civilian casualties could heighten a sense of desperation among Syrians and provide a recruiting tool for extremist groups like IS.
"It's a fight like we haven't seen before," he said.