The Islamic State group has ousted a US-backed coalition of Kurdish and Arab forces from its holdout in eastern Syria, killing dozens of fighters, a monitoring group said Sunday.
A Syrian Democratic Forces commander, asking not to be named, confirmed the SDF retreat from the Hajin pocket near the Iraqi border seven weeks into an offensive.
The SDF, who are backed by air strikes of the US-led coalition, launched its campaign to retake the IS holdout on September 10.
But they have faced a fierce fightback from the jihadists, including under the cover of sandstorms, the Britain-based Syrian Observatory for Human Rights says.
"In counterattacks since Friday to Sunday dawn, IS has taken back all positions to which the SDF had advanced inside the Hajin pocket," the monitoring group's chief Rami Abdel Rahman said.
The Observatory reported 72 SDF fighters killed, as IS took advantage of the storm that hampered coalition air cover and dispatched suicide bombers as part of their fightback.
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The SDF commander told AFP that his forces had faced a "strong dust storm" and lacked local knowledge of the terrain.
Unlike IS, "our forces don't know the area and can't move around in conditions of zero visibility," he said.
"Military reinforcements and heavy weapons have been sent to the front and some units will be replaced by more experienced ones," the commander said.
"We will launch a new military campaign as soon as those reinforcements have arrived," he said.
More than 300 SDF fighters and around 500 IS jihadists have been killed in the past seven weeks of fighting, the Observatory says.
The coalition estimates that 2,000 IS fighters remain in the Hajin area. IS overran large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" across land it controlled.
But the jihadist group has since lost most of that territory to various offensives in both countries.
In Syria, its presence has been reduced to parts of the vast Badia desert also in the east and the Hajin pocket.
A total of more than 360,000 people have been killed since Syria's war erupted in 2011 with the brutal repression of anti-government protests.