BEIRUT (Reuters) - Islamic State militants attacked U.S.-backed Syrian Democratic Forces (SDF) east of Raqqa on Friday, setting off clashes and abducting a number of people, the Syrian Observatory for Human Rights said.

The Observatory said the clashes had resulted in casualties among displaced people and SDF fighters, but gave no further details. The targeted area, al-Karama, hosts a camp for Syrians driven from their homes by the war. A Kurdish official confirmed the report but gave no more details.

The SDF is dominated by the Kurdish YPG militia, and is the main partner for the U.S.-led coalition against Islamic State in Syria. It is currently battling the jihadists for control of Raqqa city.

The Observatory says the SDF has captured half of the city.

Islamic State has lost swathes of territory in Syria over the last year to separate campaigns waged by the SDF, the Russian-backed Syrian military, and Turkey-backed Syrian rebels.