IS late Friday dispatched suicide bombers as part of a counter-attack against the Syrian Democratic Forces, 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, using an alternative acronym for IS.
In a statement on the Telegram messaging app, IS 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.
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