Syrian Democratic Forces spokesman Mustafa Bali tweeted that the offensive began Saturday after more than 20,000 civilians were evacuated from the ISIS-held area in the eastern province of Deir el-Zour.
Syrian opposition activists reported that Islamic State militants have attacked U.S.-backed fighters near an oil field in the country's east, triggering airstrikes by the U.S.-led coalition.
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The Britain-based Syrian Observatory for Human Rights says 12 ISIS gunmen attacked the Kurdish-led Syrian Democratic Forces and clashed with them for several hours until most of the attackers were killed early Saturday.
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Other activist collectives, including the Step news agency, reported the attack, saying some of the attackers used motorcycles rigged with explosives.
The fighting was concentrated near al-Omar field, Syria's largest.
The Syrian Democratic Forces launched its offensive to end the ISIS presence in Syria in September, a battle that left hundreds of fighters dead on both sides. The Syrian Democratic Forces has driven ISIS from large swaths of territory it once controlled in northern and eastern Syria, confining the extremists to a small pocket of land near the border with Iraq.
U.S. President Donald Trump predicted Wednesday that ISIS will have lost all the territory it once controlled in Iraq and Syria by next week.