Twenty-four members of the US-backed Syrian Democratic Forces (SDF) were killed by ISIS in two days in an attack in eastern Syria, said the Syrian Observatory for Human Rights.
The SDF is battling to expel the extremists from a pocket in the eastern province of Deir Ezzor on the Iraqi border.
The Britain-based Observatory said ISIS broke out of that pocket on Friday to attack a nearby village where SDF fighters and coalition advisers are based.
"ISIS launched a broad attack on the village of Al-Bahra next to its holdout, taking advantage of the fog," Observatory chief Rami Abdel Rahman said.
Abdel Rahman said 24 SDF fighters were killed in the attack, and violent clashes continued to rage on Saturday as coalition air planes carried out air strikes in the area.
Twenty-seven ISIS members were killed in clashes and air strikes in the same period, according to the Observatory, which says it relies on sources inside Syria for its information.
The monitor said coalition raids has also killed 17 civilians, including five children, in the ISIS-held pocket since Friday.
Coalition spokesman Sean Ryan said he had not received any reports of civilian casualties, and insisted its 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 targeting non-combatants.
ISIS overran large swathes of Syria and neighboring Iraq in 2014, proclaiming a "caliphate" in land it controlled but has lost most of it to offensives by multiple forces in both countries.
In Syria, the terrorists are largely confined to the pocket in Deir Ezzor, but they also have a presence in 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 put the number 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-regime protests.