It said the Mayadeen mission was “meticulously planned and executed to reduce the risk.to non-combatants”.
The United States is reviewing allegations from an independent group in Syria after the group said a USA -led coalition airstrike on an Islamic State-run jail in eastern Syria killed almost 60 people, many of them believed to be civilian prisoners.
The White House on Monday accused Assad’s regime of preparing a potential chemical attack and warned it would pay a “heavy price”, but Defence Secretary Jim Mattis said Washington was determined to keep a strict focus on fighting ISIS.
Washington launched a retaliatory cruise missile strike days later against Syria’s Shayrat airbase from where it said the chemical attack was launched, the first direct U.S. action against the regime.
The UK-based observatory said the air strike took place on Monday at dawn, hitting a building in the town of al-Mayadeen that was being used as an Islamic State prison.
Syrian activists say an airstrike targeting an Islamic State-run jail in eastern Syria has killed at least 42 prisoners.
It was reported that the town was once the home of a commander of al-Nusra Front, an al-Qaeda-linked group now known as Hayat Tahrir al-Sham.
The activist-run Deir Ezzor 24 media outlet said at least 60 civilians were killed.
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 regime army and its Russian ally.
Col. Ryan Dillon, a United States military spokesman in Baghdad, said the coalition had struck command-and-control facilities and other infrastructure belonging to the Islamic State near Mayadeen on Monday.
The White House statement on Monday night was made without forewarning and caught State Department officials by surprise.