"We never thought or said this fight would be easy".
The withdrawal likely came "under a deal with the regime forces" after weeks of encirclement and air raids, he said.
Following the setback, hundreds of Kurdish fighters were deployed to the area's outskirts as reinforcements.
Arab media announced the deaths of several dozen people, most of whom appeared to have been Islamic State group fighters, during bitter fighting in the Deir el-Zor region of eastern Syria, not far from the Iraqi border.
The fighting is the latest phase of efforts by the US-led coalition and the Syrian Democratic Forces (SDF) to clear Islamic State from its last foothold east of the Euphrates River following last year's defeat of the group in Raqqa, its Syrian headquarters.
The Observatory said coalition air strikes early Saturday on the village of Abu al-Husn in Deir Ezzor province killed 43 people, including 36 family members of IS fighters.
"It's the highest death toll in coalition air strikes since the SDF launched its attack against the IS pocket" in September, Observatory chief Rami Abdul-Rahman said.
On Friday, media reports said Jabhat al-Nusra and groups affiliated to the terrorist outfit were preparing to use chemical weapons against civilians and blame the attack on the government of President Bashar al-Assad in a bid to justify a new Western military aggression.
Another 7 victims had not yet been identified as either civilians or ISIS fighters, it said.
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The IS-linked Aamaq news agency also reported 40 killed, quoting a medical official in the Hajin area.
The SDF assault was slowed by a fierce militant fightback, and then briefly put on hold to protest Turkish shelling of Kurdish militia positions in northern Syria.
"There has been an advance on the ground in the past days but it is a careful advance due to fields of landmines, trenches, tunnels and barricades set up by IS", he told AFP.
ISIS overran large swathes of Syria and neighboring Iraq in 2014, proclaiming a "caliphate" in land it controlled.
But the militant group has since lost most of it to various offensives in both countries.
That fighting has killed 240 regime fighters and 420 IS militants, the Observatory says.
The province is the heartland of the country's Druze minority, which made up roughly three percent of Syria's pre-war population - or about 700,000 people.
Followers of a secretive offshoot of Islam, the Druze are considered heretics by the Sunni extremists of IS.
Syria's war has killed more than 360,000 people since it erupted in 2011 with the repression of anti-government protests.