Die-hard jihadists defending their last bastions in eastern Syria used the cover of bad weather to launch a vain but deadly counterattack against Kurdish-led fighters.
The Islamic State group was unable to hold on to the positions they attacked but the assault killed 23 members of the US-backed Syrian Democratic Forces (SDF) and also left nine jihadists dead.
IS fighters took advantage of poor visibility to unleash suicide attackers on SDF forces along the front line in the Euphrates valley late on Sunday, the Syrian Observatory for Human Rights said Tuesday.
"Twenty-three SDF fighters were killed and nine IS jihadists were also killed in fighting that lasted all night and into Monday morning," Observatory chief Rami Abdel Rahman said.
The jihadists often launch attacks under the cover of bad weather that cancels out their opponents' advantage of US-led coalition air power.
The SDF launched what is meant to be the final offensive on the jihadist organisation four months ago with air and ground support from coalition forces.
The Kurdish-Arab alliance has deployed some 17,000 fighters for an operation aimed at flushing out IS from the last rump of its now-defunct "caliphate".
IS fighters "launched deadly counterattacks in three different directions against the Syrian Democratic Forces, including in the villages of Sousa and Al-Shaafa," Abdel Rahman said.
He said they used at least two suicide bombers in their attacks, which inflicted the latest in a string of heavy losses on the SDF.
According to the Observatory, 1,087 IS jihadists were killed since the start of the operation on September 10 while 602 members of the SDF also died.
"On Monday morning, the SDF launched an offensive and retook all the positions they lost," the Observatory said.
"Due to its depleted manpower, IS was unable to hold on to the positions it attacked," it said.
Abdel Rahman said the jihadists' defences in the area have collapsed and the end of the battle is near.
The IS jihadists who remain however include seasoned fighters who have little to lose and are prepared to die in a last stand.
The meanders of the Euphrates in those areas of the Deir Ezzor province near the border with Iraq are considered the heartland of IS and are a perilous terrain for the SDF.
In mid-December, the SDF took Hajin, the last town of note in the IS-controlled pocket, signalling the imminent fall of the jihadists' last bastion.
An announcement by US President Donald Trump last month that he was ordering a complete troop pullout from Syria rattled the Kurds.
It left them exposed to the threat of a cross-border operation by their archfoe Turkey and protesting that they had been poorly rewarded for doing much of the heavy-lifting in the battle against IS.
They have pressed on with their operation in eastern Syria regardless and Washington has since stressed any withdrawal would be gradual.
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