The international coalition fighting the Islamic State group has opened a formal investigation to determine whether its air strikes last week near the Syrian city of Manbij claimed civilian lives, a spokesman said Wednesday.

The main Syrian opposition group had urged the US-led coalition to suspend its bombardments following the 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 worst of the bombardments, according to local sources, took place on July 19. The Britain-based Syrian Observatory for Human Rights said 56 civilians died as they fled from a village near Manbij, a strategic waypoint between Turkey and the jihadist stronghold of Raqa.

Garver had earlier accused the Islamic State of using civilians as "human shields."

After the Manbij bombardment, Amnesty International had urged the coalition to redouble its efforts to prevent civilian deaths and to investigate possible violations of international humanitarian law.

It said the Manbij attack might be the costliest by the coalition in civilian lives lost since it began anti-jihadist operations in Syria in 2014.

The London-based nongovernmental organization 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.

Garver said last week that the jihadists had been mounting exceptionally fierce resistance in Manbij.

"It's a fight like we haven't seen before," he said.