BEIRUT — The Islamic State killed at least 40 U.S.-backed Syrian fighters, captured several alive and regained areas they lost earlier this month in eastern Syria near the Iraqi border in some of the most intense fighting in weeks, a war monitor and an agency linked to the Islamic State said Saturday.

Members of the U.S.-backed Syrian Democratic Forces have been on the offensive since early September under the cover of airstrikes by the U.S.-led coalition to capture the last pocket held by the Islamic State in Syria.

Friday’s fighting that lasted until the early hours of Saturday began when Islamic State fighters, taking advantage of a sandstorm, launched a counteroffensive against Syrian Democratic Forces positions east of the Euphrates river in the eastern province of Deir el-Zour that borders Iraq, activists said.

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