Counter-attacks by ISIS killed at least 47 US-backed fighters over two days as the extremists struck from their besieged holdout in eastern Syria.
The Syrian Democratic Forces, a Kurdish-led alliance supported by the US, is battling to expel ISIS from the eastern province of Deir Ezzor on the Iraqi border.
The Britain-based Syrian Observatory for Human Rights said the militants launched three separate assaults on Saturday.
It said the counter-attacks were against the villages of Al Bahra and Gharanij and an area close to the Al-Tanak oilfield, which is commercially active but is also an SDF military position.
SDF spokesman Mustefa Bali confirmed "a series of attacks" led by ISIS in these three locations. He said fighting took place all day, with SDF ground forces receiving US-led coalition air support.
Saturday's fighting alone killed 29 SDF fighters, taking its total losses over the last two days to at least 47, said Observatory head Rami Abdel Rahman.
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Thirty-nine ISIS extremists were killed over the same period, the Observatory said. It also said coalition raids killed 17 civilians, including five children, in ISIS-held territory since Friday.
However, coalition spokesman Sean Ryan said he had not received any reports of civilian casualties, and insisted air strikes had been "very limited due to the weather".
The anti-ISIS alliance has repeatedly denied previous reports of civilians killed in its air strikes, and said it does its utmost to avoid hitting non-combatants.
Deir Ezzor activist Omar Abu Leila said the attack on Al Bahra was "very scary" and that IS fighters were able to move quickly, "taking advantage of the fog".
ISIS overran large parts of Syria and neighbouring Iraq in 2014, proclaiming a so-called caliphate in land it controlled. Since then, it lost most of this territory.
In Syria, ISIS is largely confined to Deir Ezzor, but it also has a presence in the vast Badia desert that stretches across the country to the Iraqi border.
Since 2014, the US-led coalition has acknowledged direct responsibility for over 1,100 civilian deaths in Syria and Iraq, but rights groups say the true number is much higher.
Syria's war has killed more than 360,000 people and displaced millions since it started in 2011 with the brutal repression of anti-government protests.
