The US-led coalition killed at least 100 regime and allied fighters in retaliation for an attack in eastern Syria, a US official said, in one of the deadliest incidents of its kind.
The flare-up lasted into the small hours of Thursday and was sparked by an attack on positions held by the Kurdish-led Syrian Democratic Forces (SDF) in Deir Ezzor province.
According to the US Central Command, coalition advisers were also present at the site of the attack late on Tuesday.
"The coalition conducted strikes against attacking forces to repel the act of aggression," it said.
The SDF and the coalition targeted the attacking forces with air and artillery strikes after "20 to 30 artillery and tank rounds landed within 500 metres (yards) of the SDF headquarters location," the official said.
The town lies along the Euphrates river, southeast of the provincial capital of Deir Ezzor.
"Regime forces struck SDF positions in several villages and an oil field with artillery fire," Rami Abdel Rahman, the head of the Britain-based monitoring group, said.
"The SDF responded with artillery fire on regime positions in the town of Khasham before the intervention of coalition forces," he said.
Regime and SDF fighters were involved in several skirmishes in the area last year, as they each conducted parallel operations against some of the Islamic State jihadist group's last bastions.