Strikes by a US-led coalition against the IS group killed at least 15 civilians, including three children, in northeastern Syria on Thursday, a monitor said.
The strikes hit four IS-group controlled villages in Hasakeh province, the Syrian Observatory for Human Rights said, saying the toll could rise.
It said nine IS group militants had also been killed in the strikes on the four villages.
The Britain-based monitor said the latest civilian casualties came after two days of civilian deaths in US-led strikes in Hasakeh.
It reported another eight civilians, including one woman, were killed in US-led strikes on another IS- group controlled village in the province on Wednesday.
Strikes elsewhere in the province also killed 26 IS group militants on the same day, it said.
And on Tuesday, 15 civilians were killed in US-led strikes on the town of Al-Shadadi, the group said.
That brought the number of civilians reported killed in US-led strikes in Hasakeh to 38 since Tuesday.
The coalition has reportedly stepped up its raids in Hasakeh since the Kurdish-led Syrian Democratic Forces alliance launched a new operation against IS group in the southern part of the province on Tuesday.
The alliance has scored successes against IS group in other parts of the province in the past, backed by the coalition which began strikes in Syria in September 2014.
It has rarely acknowledged civilian deaths in its campaign against IS group.
In January, it admitted three "likely" civilian deaths last year near the IS group stronghold of Raqa, bringing to at least 21 the number of civilians the coalition has acknowledged killing in both Syria and neighbouring Iraq.
The Observatory said last month that the coalition campaign had killed some 332 civilians in Syria since it began, among them 90 children.
In total, the campaign has killed more than 4,250 people in Syria, according to the monitor, which relies on a network of sources on the ground.