The US-led coalition killed at least 100 Syrian government and allied fighters over Wednesday night in response to what it said was an “unprovoked attack” in eastern Syria, a US official said, in one of the deadliest incidents of its kind.

The flare-up lasted into the early hours of Thursday and was sparked by an attack on positions held by the Kurdish-led Syrian Democratic Forces (SDF) in Deir Ezzor province, the Combined Joint Task Force said.

The clash came against a backdrop of escalating tensions between Washington and Damascus over an increase in the suspected use of chemical weapons by the government and allied militia.

Statements carried by Syrian television stations called the bombing a “new aggression” and “an attempt to support terrorism” with a reporter describing those killed as “local people fighting (Islamic State) and the SDF”.

The US Central Command said that the coalition “conducted strikes against attacking forces to repel the act of aggression”.

“We estimate more than 100 Syrian pro-government forces were killed while engaging SDF and coalition forces,” while one SDF member was wounded, a US military official said on condition of anonymity.

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.

According to the Syrian Observatory for Human Rights, which only confirmed 20 dead among government and allied forces, the initial attack took place near Khasham.

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.

Government 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 militant group’s last bastions.