A rights group says a U.S.-led air strike in the northern Syrian province of Aleppo on May 1 killed 52 people, including seven children.
The U.K.-based Observatory for Human Rights said on May 2 that the air raid mistakenly struck civilians in a village on the eastern banks of the Euphrates River and killed members of at least six families.
The group said at least 13 people are still missing.
The report could not be independently confirmed.
U.S. and Arab forces have been carrying out almost daily air raids against hard-line Islamist militant groups in Syria such as Islamic State since September 2014, and U.S.-led forces are also targeting the group in Iraq.
The United States says it takes reports of civilian casualties from the U.S.-led strikes seriously and investigates each allegation.