BEIRUT -- An independent group that monitors the war in Syria said Tuesday that a likely U.S.-led coalition airstrike on an ISIS-run jail in the country's east had killed at least 57 people, many of them believed to be civilian prisoners held by the terrorist group.

According to the London-based Syrian Observatory for Human Rights (SOHR), which relies on a network of activists inside Syria for its information and generally proves accurate, "warplanes, most likely belonging to the U.S. coaltion," destroyed an ISIS compound in Deir Ezzor province early Tuesday morning.

The U.S.-led anti-ISIS coalition, which has been accused repeatedly this year of causing civilian casualties as it targets the terror group's rapidly shrinking territory in both Syria and Iraq, acknowledged an operation in the al-Mayadeen suburb of Deir Ezzor city on Tuesday, but said in a statement quoted by the Reuters news agency that it had been "meticulously planned" to reduce the risk of civilian casualties.

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SOHR and a second activist group, the Deir Ezzor 24 online media outlet, said the building targeted in al-Mayadeen, in the southern countryside of Deir Ezzor province, once belonged to an al Qaeda-linked commander before being seized by ISIS in 2014.

About 15 of those killed in Tuesday's strike were said to have been ISIS militants, but even they could have been prisoners of the group -- accused of treason or any of the other myriad offenses the militants regularly persecute civilians and their own members for.

Eyewitnesses said ISIS fighters were seen bringing victims' bodies out of the rubble and laying them on the side of a road. There were about 100 detainees in the jail when it was hit, and SOHR said the death toll would likely rise due to the number of people seriously wounded in the attack.

There has been a dramatic increase in reported civilian deaths resulting from U.S. and coalition airstrikes in both Syria and Iraq since the beginning of the year.

Earlier this month, a top United Nations human rights official said the U.S.-led coalition had caused a "staggering loss of civilian life" with airstrikes around the Islamic extremists' remaining stronghold in the Syrian city of Raqqa.

"We note in particular that the intensification of airstrikes, which have paved the ground for an SDF (Syrian Democratic Forces) advance in Raqqa, has resulted not only in staggering loss of civilian life, but has also led to 160,000 civilians fleeing their homes and becoming internally displaced," said Paulo Pinheiro, chairman of U.N.'s Independent Commission of Inquiry on the Syrian war.

Pinheiro told the U.N.'s Human Rights Council in Geneva that, "the imperative to fight terrorism must not, however, be undertaken at the expense of civilians who unwillingly find themselves living in areas where ISIL (ISIS) is present."

U.S. military officials insist there has been no change in the rules of engagement for American forces operating in those two countries against ISIS, and say higher civilian casualties were always expected as the militants are increasingly squeezed into shrinking territory.