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 IS 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 monitor said 14 civilians and nine IS jihadists were killed Saturday in coalition air strikes in Hajin and the nearby towns of Sousa and Al-Shaafa.
A coalition spokesman said those strikes targeted IS positions.
"The strikes killed ISIS terrorists and destroyed three operational facilities critical to ISIS's operations," Sean Ryan told AFP, using an alternative acronym for IS.
The SDF, with the support of coalition air strikes, in September launched an offensive to wrest the Deir Ezzor pocket including Hajin from IS, making slow advances.
But the alliance suffered a major setback as they retreated last week from the entire pocket after IS 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 2,000 IS fighters remain in the Hajin pocket near the border with Iraq, which has reinforced military positions near its Al-Qaim crossing to prevent a spillover of clashes.
IS overran large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" across land it controlled.
But the jihadist group has since lost most of that territory to various offensives in both countries.
In Syria, its presence has been reduced to parts of the vast Badia desert and Hajin.
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.