An airstrike targeting an Islamic State-run jail in eastern Syria on Monday killed at least 42 prisoners, Syrian activists said.
At least 15 ISIS jailers and fighters were also killed in the airstrike in Deir El-Zour province, along with at least 60 civilians, Britain-based Syrian Observatory for Human Rights and activist-run Deir Ezzor 24 media outlet reported.
ISIS seized the building in 2014. It had previously belonged to an al-Qaida-linked commander, according Deir Ezzor 24 media outlet.
It's unclear who carried out the strike, though two groups have said the U.S.-led coalition was responsible. The coalition could not be immediately reached for comment.
The airstrike comes as the The White House issues a warning to Syrian President Bashar Assad late Monday, claiming the U.S. had "potential" evidence that Syria was preparing for another chemical weapons attack.
"The United States has identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children," Press Secretary Sean Spicer said in a statement.
"Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price," Spicer warned.
Spicer referenced activities by the Assad regime that were similar to preparations taken before an April 2017 attack that killed dozens of men, women and children. Victims showed signs of suffocation, convulsions, foaming at the mouth and pupil constriction.
Assad had denied responsibility for the attack.
Trump then carried out a cruise missile strike on a Syrian government-controlled air base where U.S. officials said the Syrian military had launched the deadly chemical attack.
The Associated Press contributed to this report.