US-led coalition air strikes on the last jihadist pocket in Syria on Monday killed 16 civilians, including at least seven children, a war monitor said.
Eight women and one elderly man were also among the civilians killed while trying to flee towards the Iraqi border, said Rami Abdel Rahman, the head of the Britain-based Syrian Observatory for Human Rights.
The coalition was not immediately available for comment, but has repeatedly said it does its utmost to avoid targeting civilians.
The Kurdish-led Syrian Democratic Forces (SDF) on Saturday announced the final push to expel hundreds of diehard jihadists from their last patch of territory in eastern Syria on the Iraq border.
The SDF estimates that up to 600 IS fighters could still remain inside, most of them foreigners.
Nineteen IS fighters were killed in clashes with the SDF on Monday, the monitor said.
The Kurdish-led force lost nine fighters during battles with jihadists, the Observatory said.
The SDF does not usually divulge casualty figures until weeks or months later.