The US-led coalition has attacked a convoy full of civilians fleeing the ISIS capital of Raqqa, hitting them near the towns of Ratleh and Kasrah, and killing at least 20 people, according to reports from the Syrian state media.

This is the latest in a flurry of attacks by US forces against targets in the eastern half of Syria, centered roughly around Raqqa and other ISIS-held territory, with well over 100 civilians killed in previous strikes, most of them family members of ISIS fighters.

While the exact identities of who these civilians were in the latest strikes, they probably weren’t ISIS family members, because they were fleeing ISIS territory. This makes the incident particularly problematic from the US perspective, even if officially they have yet to address it, and Defense Secretary James Mattis is downplaying concerns about killing civilians in general.

US designs on eventually having their allies, the Kurdish YPG, invade Raqqa are going to depend heavily on their ability to get at least some of the civilians out of the city, to keep civilian casualties limited. Those evacuations will be hard to push if the US is attacking those fleeing.