U.S. military officials said coalition aircraft had accidentally struck Syrian government troops Saturday during a raid against Islamic State fighters.
Syrian and Russian sources said dozens of soldiers were killed. The Syrian Observatory for Human Rights monitoring group gave a toll of at least 80 soldiers killed.
The U.S. military's Central Command said in a statement that the strike was in an area the coalition had attacked in the past, and that coalition members had informed their Russian counterparts before it began.
The statement said the strike was halted immediately when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military.