US-backed forces pictured near the village of Susah in the eastern province of Deir Ezzor, near the Syrian border with Iraq (AFP)
The US-led coalition fighting in Syria has hit back at reports that its air strikes on an Islamic State (IS) holdout in eastern Syria had killed civilians, appearing to blame their deaths on government forces.
The Syrian Observatory for Human Rights monitor, a UK-based activist group, said coalition strikes on Saturday killed 43 people, including 36 family members of IS fighters in the village of Abu al-Husn in Deir Ezzor province.
But the coalition on Sunday denied that its air raids there had killed any non-combatants.
Brett McGurk, the US envoy for the coalition, appeared to blame government forces stationed "across the river" for the civilian casualties.
"Reports of civilian casualties attributed to coalition strikes are false. All other forces should cease uncoordinated fires from across the river immediately," he said on Twitter.
In a statement late on Saturday, the coalition reported 19 coalition strikes on IS targets "free of civilian presence" between late Friday and Saturday afternoon in the IS enclave, which includes the town of Hajin.
The coalition's "initial assessment following the strikes is that there was no evidence of civilians near the strikes," it said.
But the coalition "detected a total of ten additional strikes in the same area of Hajin that did not originate from the coalition or partner forces", it added.
It called "on all other actors to cease uncoordinated fires across the Euphrates".
The Observatory said government forces and IS fighters exchanged fire across the river on Saturday, but pro-government shelling did not hit Abu al-Husn.
IS overran large areas of Syria and neighbouring Iraq in 2014, declaring a "caliphate" in territory it controlled, but has since lost most of that territory to various offensives.
In war-torn Syria, multiple offensives have now whittled down territory IS once controlled to parts of the vast Badia desert and the small pocket in the eastern province of Deir Ezzor on the Iraqi border.
A Kurdish-led alliance backed by the coalition is battling to expel IS from that holdout on the eastern bank of the Euphrates, while Russian-backed government forces have been fighting the group west of the river.
'Fields of landmines'
The Observatory says it obtains its information from sources inside Syria, and determines who carries out air strikes according to type, location, flight patterns and munitions involved.
The coalition has consistently denied reports by the Observatory in recent weeks that its air raids have killed civilians.
It says it takes allegations of civilian casualties seriously and investigates each one thoroughly.
Since 2014, the US-led coalition has acknowledged direct responsibility for over 1,100 civilian deaths in Syria and Iraq, but rights groups put the number much higher.
The Kurdish-led Syrian Democratic Forces (SDF), who are backed by the coalition air strikes, launched an assault to seize the eastern pocket of Hajin from IS in September.
The SDF assault was slowed by a fierce IS fightback and then briefly put on hold as part of a protest against Turkish shelling of Kurdish militia positions in northern Syria.
An SDF commander on Saturday said his forces were advancing cautiously due to "fields of landmines, trenches, tunnels and barricades set up by IS".
IS loses southern pocket
To the southwest, government forces regained control of a volcanic plateau between the provinces of Damascus and Sweida on Saturday after weeks of fighting.
Pro-government fighters regained control of Tulul al-Safa "after IS fighters withdrew from it and headed east into the Badia desert," Observatory chief Rami Abdel Rahman said.
The withdrawal likely came "under a deal with the regime forces" after weeks of encirclement and air raids, he said.
State news agency SANA reported government forces had made "a great advance in Tulul al-Safa" and said they were combing the area for any remaining fighters.
Government forces had been fighting IS in the area since a deadly IS attack on the country's Druze minority in Sweida province.
In the 25 July raid, IS killed more than 260 people, most of them civilians, in a wave of suicide bombings, shootings, and stabbings in the bloodiest assault on the Druze minority of the war.
IS fighters abducted about 30 people during the raid, some of whom died while the others were later freed.
Followers of an offshoot of Islam, the Druze are considered heretics by IS.
Syria's war has killed more than 360,000 people since it started in 2011 with the brutal repression of anti-government protests.