ISIS killed 12 US-backed fighters in a surprise attack Sunday from the militants' holdout in eastern Syria on the Iraqi border.
Twelve fighters of the Syrian Democratic Forces (SDF) were killed and 20 wounded in a suicide car bombing and subsequent clashes in the eastern province of Deir Ezzor, the Syrian Observatory for Human Rights said.
An SDF spokesman, however, denied any members of his Kurdish-led alliance had been killed.
"There are counter-attacks every day and the clashes are ongoing, but the talk of martyrs among our ranks is not true," Mustefa Bali said.
According to Observatory chief Rami Abdel Rahman, Sunday's attack "started with a car bomb driven by a suicide attacker against an SDF position between Hajin and Al-Bahra."
The attack allowed ISIS to advance towards Al Bahra from its holdout around Hajin, and push back the first lines of defence of the SDF, which is backed by the US-led coalition, the Observatory said.
The offensive comes one day after US-led coalition airstrikes on ISIS positions in eastern Syria killed at least 14 civilians.
"Fourteen civilians, including five children under the age of 18, were killed in the coalition air raids on the villages of Hajin, Sousa and Al-Shaafa" in eastern Deir Ezzor province, the Observatory said.
At least 9 ISIS militants were killed in the raids, Mr Abdel Rahman said.
The coalition was not immediately available for comment.
The SDF, with the support of coalition air strikes, in September launched an offensive to wrest the Deir Ezzor pocket including Hajin from ISIS, making slow advances.
But the alliance suffered a major setback as they retreated last week from the entire pocket after ISIS suicide bombings and low visibility due to sand storms.
Last Wednesday, the SDF suspended its fight against the jihadists after Turkish forces fired on the group's positions in northern Syria.
The coalition estimates that 2,000 ISIS fighters remain in the Hajin pocket.
A total of more than 360,000 people have been killed since Syria's multi-front war erupted in 2011 with the brutal repression of anti-government protests
