The jihadist IS group overran large swathes of Syria and neighbouring Iraq in 2014, declaring a "caliphate" in territory it controlled, but has since lost most of it to various offensives.

In war-torn Syria, multiple offensives have now whittled down territory IS once controlled to a 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 regime forces have been fighting the jihadists west of the river.

The Syrian Observatory for Human Rights monitor said coalition strikes on Saturday killed 43 people, including 36 family members of IS fighters in the village of Abu al-Husn in the jihadist pocket.

But the coalition denied that its air raids there had killed any non-combatants.

The US envoy for the coalition, Brett McGurk, on Sunday appeared to blame regime forces stationed "across the river" for bombarding the area.

"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 Saturday, the coalition reported 19 coalition strikes on IS targets "free of civilian presence" between late Friday and Saturday afternoon in the jihadist enclave, which includes the town of Hajin.

- 'Other actors' -

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 regime forces and IS fighters exchanged fire across the river on Saturday, but pro-government shelling did not hit Abu al-Husn.

The Britain-based war monitor 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 US-led international coalition has consistently denied reports by the Observatory in recent weeks that its air raids have killed civilians.

It says it investigates allegations of civilian casualties thoroughly.

Syria's war has killed more than 360,000 people since it started in 2011 with the brutal repression of anti-government protests.

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, who are backed by the coalition, launched an assault to seize the eastern pocket around Hajin from IS in September.

The SDF assault was slowed by a fierce jihadist fightback, and then briefly put on hold to protest Turkish shelling of Kurdish militia positions in northern Syria.

An SDF commander Saturday said his forces were advancing cautiously due to "fields of landmines, trenches, tunnels and barricades set up by IS".

- IS loses southern pocket -

On another front, regime forces on Saturday regained control from IS of a volcanic plateau in the south of the country after weeks of fighting.

Pro-government fighters took over Tulul al-Safa between the provinces of Damascus and Sweida "after IS fighters withdrew from it and headed east into the Badia desert", Observatory chief Rami Abdel Rahman said.

State news agency SANA reported regime forces had made "a great advance in Tulul al-Safa" and were combing the area for any remaining jihadists.

Government forces had been fighting IS in Tulul al-Safa since a deadly jihadist attack against the country's Druze minority in Sweida province on July 25.

In the deadliest attack against the Druze in the seven-year war, IS killed more than 260 people, most of them civilians, in suicide bombings, shootings, and stabbings.

Saturday's victory in Tulul al-Safa leaves IS contained in the Deir Ezzor pocket, although it also has a presence in the vast Badia desert stretching across the country to the Iraqi border.

After pushing back the jihadists from parts of northeastern Syria, analysts say the SDF is also likely to oust IS eventually from its eastern holdout.

"IS does not have great chances to remain in control of the pocket of Hajin," said Julien Theron of the Paris Institute of Political Studies.

The coalition-SDF alliance "has already shown a great efficiency against IS-held territory in the recent past", he said.