The Syrian Observatory for Human Rights said on Friday that the death toll from an air strike in eastern Syria overnight had climbed to 54, including Islamic State fighters and 28 civilians.
The air strike hit an area a few kilometres (miles) from the border with Iraq. Most of the civilians were Iraqi, Observatory Director Rami Abdulrahman said by phone.
He said the air strike had most likely been carried out either by the U.S.-led coalition against Islamic State or Iraqi forces that are working with it.
Earlier reports by the Syrian state news agency SANA and the pro-Damascus al-Watan newspaper accused the U.S.-led coalition of carrying out the air strike.
The U.S.-led coalition could not be reached by phone and did not immediately respond to an emailed request for comment.
Reuters could not independently verify the report