At least 15 people were killed by a suspected US-led air strike on al-Raqqa, a monitor group reported.
"The warplanes, believed to be from the US-led anti-terror coalition, struck areas in the town of Dahham in al-Raqqa on Friday," Xinua news agency quoted Syrian Observatory for Human Rights as saying.
The UK-based watchdog group also said that the death toll could likely rise.
This is not the first US-led air strike that lead to civilian casualties.
The United States has admitted before that civilian victims may have fallen during the coalition's air strikes, which had started targeting the IS positions in Syria since 2014.
In January 2014, the IS took over al-Raqqa and declared it their de facto capital.
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