The Islamic State group has killed at least 68 US-backed fighters battling to oust the extremists from their eastern Syria holdout of Hajin, a war monitor said on Saturday.
ISIS dispatched suicide bombers as part of a counter-attack against the Syrian Democratic Forces (SDF) late on Friday, the Syrian Observatory for Human Rights said, raising the death toll from an earlier figure of 41.
The SDF fighters, who are backed by US-led coalition air strikes, were killed overnight, the Britain-based Observatory said.
"The death toll has increased due to the discovery of new victims on the front line and the existence of 100 wounded," said Observatory head Rami Abdel Rahman.
A spokesman for the US-led coalition said "at this time, numbers cannot be confirmed as both sides are taking casualties as this difficult fight... continues".
A "sandstorm allowed an ISIS counterattack, which was surprising given the conditions, but now the air is clear, the coalition will continue to increase air and fire support to assist our partners," spokesman Sean Ryan said.
In a statement on the Telegram messaging app, ISIS said it had attacked the village of Sousa late Friday and detonated a car bomb near the village of Al-Baghuza further south, down the Euphrates river.
The SDF last month launched an offensive against the extremists in the Hajin pocket on the eastern banks of the Euphrates, in Deir Ezzor province.
ISIS has staged a bloody fightback.
Since September 10, 270 SDF fighters and 496 ISIS extremists have been killed in the offensive, the Observatory says.
The coalition estimates that 2,000 ISIS fighters remain in the Hajin area.
Last week the Observatory said coalition air strikes had killed 41 civilians in Sousa, 10 of them children, on October 18 and 19.
But the coalition said it had targeted an ISIS command post on October 18, and denied carrying out any strikes in the area the following day.
ISIS overran large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" across land it controlled.
But the group has since lost most of that territory to various offensives in both countries.
In Syria, its presence has been reduced to parts of the vast Badia desert and the Hajin pocket near the border with Iraq.
More than 360,000 people have been killed since Syria's war erupted in 2011 with the brutal repression of anti-government protests.