At least 20 fighters from a US-backed force fighting the Islamic State group were killed Friday in an IS ambush in eastern Syria, a war monitor said.
The Kurdish-led Syrian Democratic Forces is waging an offensive around the town of Hajin in the province of Deir Ezzor, the jihadist group's last stronghold in the country's east.
"The fighters were advancing during a sandstorm, they were surrounded, IS members used explosives and opened fire," said the Syrian Observatory for Human Rights.
The US-backed SDF had been closing in on the IS pocket for months before formally launching its offensive on Monday.
Since then, 53 jihadists and 37 SDF fighters have been killed in fierce clashes, according to the Britain-based Observatory.
The Islamic State group once held nearly all of Deir Ezzor, but separate offensives last year by the SDF and Russian-backed regime forces left the jiahdists clinging to a small area of territory near the Iraqi border.
The SDF estimates IS has some 3,000 fighters in its besieged holdout, many of them foreigners.
A senior US diplomat visited Kurdish-held territory in Syria last month and pledged Washington's lasting support.
"We are prepared to stay here, as the president (Donald Trump) has made clear, to ensure the enduring defeat of ISIS," Ambassador William Roebuck said, using an alternative acronym for IS.
The Islamic State group once held swathes of territory across Syria and Iraq but has since seen its self-declared "caliphate" collapse.
The jihadists now control less than three percent of Syria and are mostly present in the vast Badiya desert, which lies between Damascus and the Iraqi border.
On Monday IS fighters killed 12 Syrian regime fighters in an ambush in the southern province of Sweida. Eight jihadists were also killed, the Observatory said.