US-led coalition air strikes have killed 26 civilians, including 14 children, in a holdout of the Islamic State group (IS) in eastern Syria, a UK-based activist group has said.

"Twenty-six IS family members, including 14 children and nine women, were killed in coalition air strikes on Friday morning on the town of Hajin," the Syrian Observatory for Human Rights said.

On Thursday, coalition raids killed another seven civilians in the nearby village of al-Shaafa, Observatory chief Rami Abdurrahman said.

Both Hajin and al-Shaafa are in a last pocket under IS control in the eastern province of Deir Ezzor near the border with Iraq.

A coalition spokesman did not immediately reply for a request for comment, the AFP news agency reported.

The coalition has been backing a Kurdish-Arab alliance fighting IS in the area.

IS overran large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" in land it controlled. But the group has since lost most of it to various offensives in both countries.

In Syria, the group has seen its presence reduced to parts of the vast Badia desert and the pocket in Deir Ezzor.

Since 2014, the US-led coalition has acknowledged direct responsibility for more than 1,100 civilian deaths in Syria and Iraq, but rights groups put the number killed much higher.

The Observatory says coalition strikes in Syria alone have killed more than 3,300 civilians.

Deadly Hama attack

Elsewhere in Syria, fighters from Hay'at Tahrir al-Sham (HTS) killed at least eight Syrian government troops near a planned buffer zone around the country's last major rebel bastion, the Observatory said on Saturday.

Hay'at Tahrir al-Sham is an alliance led by Syria's former al-Qaeda branch which is the dominant force in Idlib.

The Observatory said the attack took place late on Friday in the north of Hama province near the planned buffer zone around rebel-held territory in neighbouring Idlib.

"An assault by HTS targeted a Syrian regime position on the outskirts of the de-militarised zone" and was followed by clashes in which eight regime forces were killed, Abdurrahman told AFP. Two HTS fighters also died.

The de-militarised zone was announced by rebel-backer Ankara and Damascus ally Moscow in September to separate government troops from rebel fighters in Idlib and adjacent areas.

Under the deal, the rebels were supposed to have removed all heavy weapons from the buffer zone by 10 October but skirmishes have continued to pit regime forces against "radical" fighters and other insurgents on the ground.

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Rebel factions have said they withdrew their heavy weapons from the zone but HTS and other hardline groups have refused to pull out their fighters.

The deadly HTS assault came hours after government troops killed 23 fighters of a formerly US-backed rebel group inside the planned buffer zone.

Idlib and some adjacent areas are the last major rebel bastion in Syria, where the Russian-backed government has in recent months retaken much of the territory it had lost since the civil war erupted.

It had threatened an assault on rebel territory around Idlib, which is home to some three million people, but the truce deal struck by Russia and Turkey averted it.

Aid organisations had warned that a fully-fledged offensive on Idlib could spark the worst humanitarian catastrophe of the civil war so far.

Around half a million people are thought to have been killed and millions displaced since the conflict erupted with the brutal repression of anti-government protests in 2011.