Independent watchdog groups cite much higher numbers of civilian deaths than the figure reported by the coalition.

This month, the coalition said in a statement that it believed that it had unintentionally killed at least 484 civilians in Iraq and Syria since the start of the air campaign against the jihadists in 2014. More than 130 of those were killed in April, an increase in the monthly toll.

But Airwars, an organization that tracks civilian casualty reports through social media and other sources, said that as of June 8, the coalition had killed at least 4,118 civilians, and perhaps more than 16,000, since the start of the bombing campaign.

Neither group has finished investigating the reported strike on the prison.

The Syrian Observatory for Human Rights, which tracks the conflict in Syria from Britain with the help of contacts on the ground, said the strike early Monday had destroyed a house near the town of Mayadeen in eastern Syria that the jihadists had converted into a prison. The group said the strike had killed at least 42 civilian prisoners, 15 Islamic State guards and an unknown number of Islamic State members who had been locked up, presumably for running afoul of the group.

Another Syrian activist group, Deirezzor24, also reported the strike, saying the Islamic State had used the house to lock up hundreds of members of rebel groups who opposed the jihadists. It said that more than 60 civilians and a pair of Islamic State guards had been killed in Monday’s strike.