The report came just hours after the United Nations accused the military alliance of failing to protect civilians in the war-torn country.
The Syrian Observatory for Human Rights said the latest strikes targeted the town of al-Mayadeen on the eastern outskirts of the eastern province of Deir al-Zour.
"There were two rounds of strikes: one at Thursday night and the second after midnight, targeting buildings housing families of Daesh fighters," the Observatory's head, Rami Abdel-Rahman, told dpa
He added that most of those killed were Moroccan and Syrian nationals, who had fled to al-Mayadeen from al-Raqqa, the so-called capital of Daesh in north-eastern Syria.
Initially, the watchdog said at least 35 civilians had been killed in the strikes in al-Mayadeen.
The latest coalition bombing targeted the town's municipality building and reduced it to rubble, according to the Britain-based Observatory, which relies on a network of activists inside Syria.
There was no comment from the U.S.-led alliance.
The Observatory this week reported that between April 23 and May 23 of this year, coalition strikes had killed a total of 225 civilians in Syria.
Earlier on Friday, the UN said the U.S.-led alliance is not taking adequate care to prevent civilian deaths in Syria as it fights Daesh terrorists there.
"The same civilians, who are suffering indiscriminate shelling and summary executions by ISIL [Daesh], are also falling victim to the escalating airstrikes," UN Human Rights chief Zeid Ra'ad Al Hussein said, pointing to mounting fatalities.
Civilian casualties have been particularly high in Deir al-Zour.
Zeid also cited an airstrike that killed 23 farm workers, including 17 women, on May 14.
On the following day, air attacks on the Daesh-controlled city of Albo Kamal near the Syrian-Iraq border killed 59 civilians.
"Civilians should always be protected, whether they are in areas controlled by Daesh or by any other party," Zeid said in Geneva.
In addition, Daesh militants have been killing people in these areas on suspicion that they have provided target coordinates for U.S.-led strikes, Zeid said.
The US-led coalition air campaign against Daesh in Syria began in September 2014.