The U.S. -led coalition is supporting an alliance of Kurdish and Arab fighters in their assault on Islamic State in its de facto capital of Raqqa in northern Syria.
Fifteen ISIS militants died in the strike alongside the prisoners, which included civilians and members of rebel groups fighting ISIS, activist group Deirezzor24 said.
The White House warned late Monday that Syrian President Bashar Assad and his military would pay a "heavy price" for another chemical weapons attack.
It says the building belonged to an al Qaida-linked commander before it was seized by the Daesh group in 2014.
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Mattis' comments come as US -backed forces and militants allied with the government of Syrian President Bashar al-Assad have separately intensified efforts to control IS territory in southeast Syria and in the Euphrates River Valley. When asked by a reporter if he envisioned the entirety of the group's USA -supplied weapons would be taken from them following the end of the campaign against the Islamic State, Mattis said, "We'll do what we can".
The strikes come just a day after at least 12 civilians were killed on Monday when a suspected Russian airstrike struck the town of Qouriya in the eastern Syrian town of Deir az-Zor. "This mission was meticulously planned and executed to reduce the risk of collateral damage and potential harm to non-combatants".
"Our goal is always for zero civilian casualties", the statement said.
Observatory head Rami Abdel Rahman said 222 civilians, including 84 children, were killed in the largely Islamic State group-held province of Deir Ezzor.
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"We are very clear the use of chemical weapons by the Syrian regime is absolutely abhorrent and the last time the United States took action to deal with the aircraft and airbase from which these chemical weapons were used we fully supported their strike", Fallon told the BBC.
But Syria has been in a civil war since 2011, and many of those US -backed forces are also considered a threat by the Assad government.
Most of Deir Ezzor province is controlled by the militants and it has been the target of air strikes by both the coalition and the Syrian army and its Russian ally.
Spicer added the activities were similar to preparations taken before an April 2017 attack that killed dozens of men, women and children.
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Russian Federation and Syria have previously carried out airstrikes against the Islamic State in that region. But Mattis also noted that YPG militants were well-armed even before the USA last month made a decision to offer more specialised equipment for its urban assault on ISIL-held city of Raqqa.