The reassessment came as the United Nations warned that the fight for Hajin in Syria's eastern province of Deir ez-Zor was having a "devastating" effect on thousands of civilians.
The Syrian Democratic Forces has been locked in clashes for just over a month with ISIS holdouts in Hajin and surrounding villages, but they have been slowed down by harsh weather conditions and a ferocious jihadist fightback.
"The military operations in Hajin will take much longer than expected," said Redur Khalil, a top SDF commander.
"Daesh is benefiting a lot from weather factors, including sandstorms. They've helped it take cover from reconnaissance aircraft and other monitoring mechanisms," he told AFP, using an Arabic acronym for ISIS.
Khalil said the landscape around Hajin was vast and arid, "larger than just a pocket."
The SDF estimates some 3,000 jihadists are defending the area.
They have laid a massive network of trenches, barriers, and explosives that were "hindering" the SDF's advance, Khalil said.
"Most of the fighters in the Hajin region are foreigners with fighting experience," he added.
Hajin, which lies in a small stretch east of the winding Euphrates River, is one of ISIS's final holdouts in Syria.
The jihadist group lost its de facto Syrian capital Raqqa one year ago this week to the SDF, which has chased it out of swathes of the country's north and east.
The SDF has been advancing on Hajin since September 10 but ISIS launched a bloody fightback last week.
The ensuing clashes have killed more than 70 SDF fighters and dozens of ISIS jihadists, according to the Syrian Observatory for Human Rights war monitor.
The Britain-based Observatory said ISIS had even raided a nearby displacement camp and took "more than 100 families," including relatives of ISIS defectors and of jihadists killed in fighting.
The UN on Tuesday warned the recent fighting was endangering civilians.
"The ongoing violence in the Hajin area in the eastern governorate of Deir ez-Zor is having a devastating impact on civilians," the UN's office in Syria said Tuesday.
It estimated that around 7,000 people had been displaced by fighting over the past month to SDF-controlled camps, and that another 10,000 people were at risk.
Accessing civilians in need of aid was extremely difficult.
"The UN calls on all parties involved to protect civilians and to support greater access to the area as required under International Humanitarian Law," it said.
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