Scores of civilians trapped in Islamic State-controlled territory in northern Syria were reportedly killed Tuesday by airstrikes from Western coalition aircraft. The reported death toll, potentially the highest ever to result from a coalition bombing in the international campaign against ISIS, continued to climb as The Intercept reached out to monitoring groups tracking operations in the area.

The Syrian Observatory for Human Rights said at least 56 civilians were killed when their convoy of vehicles attempted to slip out of an area north of the city of Manbij in the predawn darkness, as U.S.-backed forces pushed forward in an increasingly bloody offensive in the area. In a brief phone interview, a representative from the Britain-based organization said that while coalition aircraft were believed to be responsible for the air raid, the group suspected it was a “100 percent mistake.”

Airwars, a nonprofit that tracks claims of civilian casualties resulting from the international air campaign against ISIS, said incoming reports indicated the death toll may prove to be well over 100 civilians — potentially making it the largest single loss of civilian life resulting from coalition airstrikes since the U.S.-led campaign to destroy ISIS began nearly two years ago. Tuesday’s reports were the latest in a string of recent incidents in which coalition aircraft have been implicated in the deaths of civilians in the Manbij area.

“Really these civilians are in a desperate situation,” Chris Woods, head of Airwars, told The Intercept. “We’ve never seen anything like this.”

The Pentagon confirmed it had reports of civilian casualties in the area around Manbij. “We are aware of reports alleging civilian casualties in the area,” U.S. Marine Maj. Adrian J.T. Rankine-Galloway, a Pentagon spokesperson, said in an emailed statement. “As with any allegation we receive, we will review any information we have about the incident, including information provided by third parties, such as the proximity of the location to CJTF [Combined Joint Task Force] airstrikes, and any other relevant information presented.”

“If the information supporting the allegation is determined to be credible, we will then determine the next appropriate step,” the spokesperson added. “We take all measures during the targeting process to avoid or minimize civilian casualties or collateral damage and to comply with the principles of the Law of Armed Conflict.”

The push to retake Manbij and surrounding villages began in earnest in late May and has combined intense fighting on the part of U.S.-backed opposition forces and more than 450 coalition airstrikes, according to the Pentagon’s count. U.S. Central Command says that coalition forces have launched at least 11 strikes in Manbij over the last 48 hours. In a press release issued Tuesday, the Pentagon reported that its largely Kurdish-led ally — referred to alternately as the Syrian Democratic Forces or the Syrian Arab Coalition — had captured an Islamic State headquarters located in a hospital in eastern Manbij.

By all accounts, life for the 70,000 civilians in the area, who remain trapped between ISIS fighters and the coalition campaign, has been hellish. Groups monitoring conditions on the ground have reported corpses decomposing in the streets and bodies left buried under the rubble of airstrikes, inaccessible due to the insecurity in the area. Airwars estimates at least 190 civilians have been killed in coalition strikes since the campaign began, including at least 39 children and 23 women.

Exactly what happened Tuesday remains unclear. While the Syrian Observatory for Human Rights and others reported civilians killed as they fled ISIS, an account published by Syria Direct, a nonprofit media organization, reported that six missiles fired by coalition forces at 3 a.m. on Tuesday morning struck a school in the village of Tokhar, a short distance northeast of Manbij, killing “anywhere from 65 to 160 people.” Sources on the ground told the outlet the “school housed displaced people from neighboring villages.” CENTCOM, which did not comment on specific allegations of civilian casualties, said its recent strikes in the area had targeted ISIS “tactical units” and “fighting positions.” The Syrian Observatory for Human Rights told The Intercept it had no information on a school being struck. The Telegraph, which reported 85 civilians killed in Tuesday’s air raid as they fled Tokhar, suggested today’s operation was the first mission launched from Incirlik Air Base in Turkey since the facility was temporarily shut down after last week’s failed coup. The base has served as a key node in the U.S. war against ISIS.

Woods, of Airwars, said Pentagon data shows roughly 98 percent of the coalition airstrikes in the Manbij campaign are overseen by the U.S., and last week was the largest number of civilian casualties since the effort began in August 2014. His organization was in the midst of preparing a report on the marked rise in civilian casualties in the area when Tuesday’s reports began to come in. Photos circulated on social media purported to show victims, including young children, being buried in mass graves.

Compared to the regime of Syrian President Bashar Assad and his allies in the Russian military, which have employed airpower in notoriously indiscriminate ways, targeting hospitals and densely populated urban areas, the U.S. coalition has generally taken great care to avoid civilian casualties, Woods said. The developments in Manbij, he said, were particularly worrying as coalition forces prepare to mount similar campaigns to retake larger cities from ISIS control. “This is the first big assault with a U.S.-backed proxy,” he said. “This bodes very badly for Mosul,” Iraq’s second largest city, which remains under ISIS control.

“Major alarm bells are ringing for us right now,” Woods added. “There’s something very, very bad happening out there at the moment.”