Coalition spokesman Col Ryan Dillon acknowledged that it had conducted strikes on "IS command-and-control elements" in the Mayadin area on Sunday.
Rami Abdelrahman confims to Newsweek that the strike killed 42 prisoners, 11 Islamic State militant group (ISIS) fighters and four ISIS guards.
In addition to the coalition, Russian Federation and Syria often carry out air strikes against IS in eastern Syria.
The jihadists, who seized control of large parts of Syria and Iraq three years ago, are under pressure in both countries.More news: Montana man is bank robber dubbed 'AK-47 bandit'
Iranian-backed forces have also drawn closer to a military base in southern Syria where the U.S.is training more forces in the fight against ISIS.
Spicer further explained that intelligence has observed certain activities which mirror preparations taken before the April 2017 attack that killed dozens of men, women and children.
CENTCOM said it will assess the allegations that civilian prisoners were killed, and it would publish the results of its assessment in its monthly civilian casualty report. The Trump administration launched missile strikes in retaliation for that attack, which it blamed on Assad.
Pentagon spokesman Navy Captain Jeff Davis said Tuesday the latest USA warning was the result of activity at the same airbase "that indicated preparations for possible use of chemical weapons".More news: Oilers sign forward Zack Kassian to 3-year extension
Syria's war began in March 2011 with anti-government protests and after a regime crackdown evolved into a complicated, multi-front war that has killed more than 320,000 people and forced millions from their homes.
Defence Secretary Jim Mattis insisted the United States was not taking sides in the Syrian conflict.
We won't fire "unless they are the enemy, unless they are ISIS", he said, using an acronym for the jihadist organisation.
The US-led coalition admitted that it had conducted airstrikes on so-called "Islamic State" (IS) targets in the Syrian town of al-Mayadeen.More news: Vehicle bomb hits bank in Kabul, killing 29
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