ISIS has forced a US-backed coalition of Kurdish and Arab forces to withdraw from its holdout in eastern Syria, as part of a concerted counterattack launched by militants two days earlier.
The Syrian Democratic Forces (SDF), who are backed by air strikes of the US-led coalition, have retreated from the Hajin pocket near the Iraqi border, the Syrian Observatory for Human Rights said on Sunday.
"In counterattacks since Friday to Sunday dawn, ISIS has taken back all positions to which the SDF had advanced inside the Hajin pocket," the monitoring group's chief Rami Abdel Rahman says.
The Observatory reported 72 SDF fighters killed, as ISIS took advantage of storms that hampered coalition air cover and dispatched suicide bombers as part of their fightback.
The SDF launched its campaign to retake the ISIS holdout in Deir Ezzor province on September 10.
But they have faced a fierce fightback from the extremists, including under the cover of sandstorms, the Britain-based Syrian Observatory for Human Rights says.
An SDF commander, asking not to be named, confirmed Sunday's retreat and said that his forces had faced a "strong dust storm" and lacked local knowledge of the terrain.
Unlike ISIS, "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.
The coalition estimates that 2,000 ISIS fighters remain in the Hajin area. More than 300 SDF fighters and around 500 ISIS jihadists have been killed in the past seven weeks of fighting, the Observatory says.
Shiite paramilitary groups are on alert on the Iraqi side of the border following ISIS’s advance, according to Step News Agency, a Syrian activist-run media outlet.
ISIS overran large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" across lands it controlled. But the extremist 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.
Separately, Turkey's military on Sunday fired artillery shells at a Kurdish militia in Syria that is backed by the United States but deemed a terrorist group by Ankara.
The shells targeted "shelters" of the Kurdish People's Protection Units (YPG) east of the Euphrates River in the Kobane region of northern Syria, Turkey’s state-run Anadolu news agency reported.
The move comes two days after President Tayyip Erdogan issued what he said was a "final warning" to those who would endanger Turkey's borders, saying Ankara was determined to focus its attention on Syrian Kurdish fighters east of the Euphrates.
There were no immediate reports of casualties, according to the Kurdish Hawar news agency.
The bombardment targeted the Zor Magar area to the west of northern Syria's Ayn Al Arab region and was aimed at preventing "terrorist activities", Anadolu reported.
Turkey considers the YPG militia a terrorist organisation and an extension of the outlawed Kurdistan Workers' Party (PKK), which has led an insurgency in Turkey for more than three decades.
Turkey carried out an offensive against YPG forces in northern Syria's Afrin region earlier this year.
The YPG took control of large areas of northeast Syria in 2012 as government forces pulled out to fight rebels in the west.