Almost 60 people have been killed in a suspected US-led coalition air strike on a prison run by so-called Islamic State in eastern Syria, activists say.
The Syrian Observatory for Human Rights said 42 prisoners and 15 militants died when the facility near Mayadin, in Deir al-Zour province, was hit on Monday.
The Deirezzor24 news website said dozens of civilians and Syrian rebel fighters were detained there.
A coalition spokesman said it was looking into the casualty reports.
Mayadeen, which lies in the Euphrates river valley about 45km (28 miles) south-east of the city of Deir al-Zour, has been targeted frequently by coalition aircraft.
US intelligence officials believe IS moved most of its leaders to the town in recent months as Iraqi government forces advanced into the Iraqi city of Mosul and a US-backed alliance of Kurdish and Arab fighters encircled the Syrian city of Raqqa.
Media caption"Welcome to Raqqa": Watch Gabriel Gatehouse's full report
The Syrian Observatory, a UK-based group that monitors the country's six-year civil war through a network of sources on the ground, said the prison was hit at dawn on Monday by what were believed to have been coalition aircraft.
Deirezzor24, which is run by an activist collective, reported that the facility was located near the village of Tayibiya wa al-Maaharufa, and that it had once been the home of a commander of al-Nusra Front, an al-Qaeda-linked group now known as Hayat Tahrir al-Sham.
Many fighters opposed to IS, including those from Tahrir al-Sham and the Western-backed Free Syrian Army, were held there along with civilians, it said.
It put the death toll at 60, but said it only included two militants serving as guards.
Coalition spokesman Col Joe Scrocca acknowledged that it had conducted strikes on "known IS command and control facilities and other IS infrastructure" in the Mayadin area on Sunday and Monday.
"The removal of these facilities disrupts [IS's] ability to facilitate and provoke terrorist attacks against the coalition, our partner forces and in our homelands," he told Reuters news agency in an email.
"This mission was meticulously planned and executed to reduce the risk of collateral damage and potential harm to non-combatants."
At the start of June, the coalition said its 21,035 air strikes in Syria and Iraq since 2014 had unintentionally killed at least 484 civilians. However, human rights groups believe the true figure is far higher.
Airwars, an organisation that tracks allegations of civilian deaths, estimates that at least 4,118 civilians are likely to have died.