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 UK-based Syrian Observatory for Human Rights said 42 prisoners and 15 militants died when the facility in the Mayadin area was hit early on Monday.
The activist-run Deirezzor24 website said the prisoners included civilians and Syrian rebels opposed to IS.
A coalition spokesman said it was looking into the reports.
The coalition has said it carried out seven strikes near the city of Deir al-Zour - about 45km (28 miles) north-west of Mayadin - on Monday.
Three of the strikes destroyed an improvised explosive device (IED) factory, an IS financial facility and an IS headquarters, according to a statement.
Another strike late on Sunday destroyed an IS-held building in the area.
Mayadin has been targeted frequently by the coalition in recent months.
US intelligence officials say IS has moved most of its leaders to Mayadin since its fighters were driven out of almost all of the Iraqi city of Mosul and were encircled in the Syrian city of Raqqa by a US-backed alliance of Kurdish and Arab fighters.