The Syrian Observatory for Human Rights said U.S.-backed fighters seized the last Islamic State enclave in eastern Syria on Saturday after the jihadists who were still there surrendered.
Reuters could not immediately confirm the report.
The UK-based war monitoring group said the last few hundred Islamic State militants, many of them foreigners, had surrendered in the past two days to the Syrian Democratic Forces. It said some militants may still be hiding in underground tunnels.
With the help of U.S. air strikes, the Kurdish-led SDF has battled to crush Islamic State in the shrinking Baghouz enclave east of the Euphrates river near the Iraqi border.