The Kurdish-Arab Syrian Democratic Forces (SDF), backed by coalition airpower, has been fighting to oust IS from the pocket in the eastern province of Deir Ezzor since September.
"Air raids and artillery fire by the international coalition and the SDF are going on against neighbourhoods of the town of Hajin," the Syrian Observatory for Human Rights said.
The air strikes "killed nine civilians, including six children, and two IS jihadists", the Britain-based monitor said.
The coalition did not immediately comment on the alleged fatalities but has repeatedly said it does its utmost to avoid civilian casualties.
The SDF broke into the key IS holdout of Hajin on Thursday after months of fighting that has seen the jihadists launch vicious counter-attacks.
He said IS losses had been "very high" and that intelligence reported jihadists had tried to flee across the border to Iraq but were prevented by Iraqi forces.
In almost three months of battle, more than 820 jihadists and more than 480 US-backed fighters have been killed, the Observatory says.
More than 300 civilians have been killed in that period, it says, though the coalition insists it does not target non-combatants.
But various offensives in both countries have routed the jihadists from most of that land, crushing their dreams of statehood.
Syria's war has killed 360,000 people and displaced millions since starting in 2011 with the brutal repression of anti-regime protests.
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